Chapter Editors: Wolfgang Laurig and Joachim Vedder
Table of Contents
Wolfgang Laurig and Joachim Vedder
The Nature and Aims of Ergonomics
William T. Singleton
Analysis of Activities, Tasks and Work Systems
Véronique De Keyser
Ergonomics and Standardization
Pranab Kumar Nag
Juhani Smolander and Veikko Louhevaara
Postures at Work
Fatigue and Recovery
Rolf Helbig and Walter Rohmert
Eberhard Ulich and Gudela Grote
Controls, Indicators and Panels
Karl H. E. Kroemer
Information Processing and Design
Andries F. Sanders
Designing for Specific Groups
Joke H. Grady-van den Nieuwboer
Antoine Laville and Serge Volkoff
Workers with Special Needs
Joke H. Grady-van den Nieuwboer
System Design in Diamond Manufacturing
Disregarding Ergonomic Design Principles: Chernobyl
Vladimir M. Munipov
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Diversity and Importance of Ergonomics - Two Examples
Definition and Scope
Ergonomics means literally the study or measurement of work. In this context, the term work signifies purposeful human function; it extends beyond the more restricted concept of work as labour for monetary gain to incorporate all activities whereby a rational human operator systematically pursues an objective. Thus it includes sports and other leisure activities, domestic work such as child care and home maintenance, education and training, health and social service, and either controlling engineered systems or adapting to them, for example, as a passenger in a vehicle.
The human operator, the focus of study, may be a skilled professional operating a complex machine in an artificial environment, a customer who has casually purchased a new piece of equipment for personal use, a child sitting in a classroom or a disabled person in a wheelchair. The human being is highly adaptable but not infinitely so. There are ranges of optimum conditions for any activity. One of the tasks of ergonomics is to define what these ranges are and to explore the undesirable effects which occur if the limits are transgressed—for example if a person is expected to work in conditions of excessive heat, noise or vibration, or if the physical or mental workload is too high or too low.
Ergonomics examines not only the passive ambient situation but also the unique advantages of the human operator and the contributions that can be made if a work situation is designed to permit and encourage the person to make the best use of his or her abilities. Human abilities may be characterized not only with reference to the generic human operator but also with respect to those more particular abilities that are called upon in specific situations where high performance is essential. For example, an automobile manufacturer will consider the range of physical size and strength of the population of drivers who are expected to use a particular model to ensure that the seats are comfortable, that the controls are readily identifiable and within reach, that there is clear visibility to the front and the rear, and that the internal instruments are easy to read. Ease of entry and egress will also be taken into account. By contrast, the designer of a racing car will assume that the driver is athletic so that ease of getting in and out, for example, is not important and, in fact, design features as a whole as they relate to the driver may well be tailored to the dimensions and preferences of a particular driver to ensure that he or she can exercise his or her full potential and skill as a driver.
In all situations, activities and tasks the focus is the person or persons involved. It is assumed that the structure, the engineering and any other technology is there to serve the operator, not the other way round.
History and Status
About a century ago it was recognized that working hours and conditions in some mines and factories were not tolerable in terms of safety and health, and the need was evident to pass laws to set permissible limits in these respects. The determination and statement of those limits can be regarded as the beginning of ergonomics. They were, incidentally, the beginning of all the activities which now find expression through the work of the International Labour Organization (ILO).
Research, development and application proceeded slowly until the Second World War. This triggered greatly accelerated development of machines and instrumentation such as vehicles, aircraft, tanks, guns and vastly improved sensing and navigation devices. As technology advanced, greater flexibility was available to allow adaptation to the operator, an adaptation that became the more necessary because human performance was limiting the performance of the system. If a powered vehicle can travel at a speed of only a few kilometres per hour there is no need to worry about the performance of the driver, but when the vehicle’s maximum speed is increased by a factor of ten or a hundred, then the driver has to react more quickly and there is no time to correct mistakes to avert disaster. Similarly, as technology is improved there is less need to worry about mechanical or electrical failure (for instance) and attention is freed to think about the needs of the driver.
Thus ergonomics, in the sense of adapting engineering technology to the needs of the operator, becomes simultaneously both more necessary and more feasible as engineering advances.
The term ergonomics came into use about 1950 when the priorities of developing industry were taking over from the priorities of the military. The development of research and application for the following thirty years is described in detail in Singleton (1982). The United Nations agencies, particularly the ILO and the World Health Organization (WHO), became active in this field in the 1960s.
In immediate postwar industry the overriding objective, shared by ergonomics, was greater productivity. This was a feasible objective for ergonomics because so much industrial productivity was determined directly by the physical effort of the workers involved—speed of assembly and rate of lifting and movement determined the extent of output. Gradually, mechanical power replaced human muscle power. More power, however, leads to more accidents on the simple principle that an accident is the consequence of power in the wrong place at the wrong time. When things are happening faster, the potential for accidents is further increased. Thus the concern of industry and the aim of ergonomics gradually shifted from productivity to safety. This occurred in the 1960s and early 1970s. About and after this time, much of manufacturing industry shifted from batch production to flow and process production. The role of the operator shifted correspondingly from direct participation to monitoring and inspection. This resulted in a lower frequency of accidents because the operator was more remote from the scene of action but sometimes in a greater severity of accidents because of the speed and power inherent in the process.
When output is determined by the speed at which machines function then productivity becomes a matter of keeping the system running: in other words, reliability is the objective. Thus the operator becomes a monitor, a trouble-shooter and a maintainer rather than a direct manipulator.
This historical sketch of the postwar changes in manufacturing industry might suggest that the ergonomist has regularly dropped one set of problems and taken up another set but this is not the case for several reasons. As explained earlier, the concerns of ergonomics are much wider than those of manufacturing industry. In addition to production ergonomics, there is product or design ergonomics, that is, adapting the machine or product to the user. In the car industry, for example, ergonomics is important not only to component manufacturing and the production lines but also to the eventual driver, passenger and maintainer. It is now routine in the marketing of cars and in their critical appraisal by others to review the quality of the ergonomics, considering ride, seat comfort, handling, noise and vibration levels, ease of use of controls, visibility inside and outside, and so on.
It was suggested above that human performance is usually optimized within a tolerance range of a relevant variable. Much of the early ergonomics attempted to reduce both muscle power output and the extent and variety of movement by way of ensuring that such tolerances were not exceeded. The greatest change in the work situation, the advent of computers, has created the opposite problem. Unless it is well designed ergonomically, a computer workspace can induce too fixed a posture, too little bodily movement and too much repetition of particular combinations of joint movements.
This brief historical review is intended to indicate that, although there has been continuous development of ergonomics, it has taken the form of adding more and more problems rather than changing the problems. However, the corpus of knowledge grows and becomes more reliable and valid, energy expenditure norms are not dependent on how or why the energy is expended, postural issues are the same in aircraft seats and in front of computer screens, much human activity now involves using videoscreens and there are well-established principles based on a mix of laboratory evidence and field studies.
Ergonomics and Related Disciplines
The development of a science-based application which is intermediate between the well-established technologies of engineering and medicine inevitably overlaps into many related disciplines. In terms of its scientific basis, much of ergonomic knowledge derives from the human sciences: anatomy, physiology and psychology. The physical sciences also make a contribution, for example, to solving problems of lighting, heating, noise and vibration.
Most of the European pioneers in ergonomics were workers among the human sciences and it is for this reason that ergonomics is well-balanced between physiology and psychology. A physiological orientation is required as a background to problems such as energy expenditure, posture and application of forces, including lifting. A psychological orientation is required to study problems such as information presentation and job satisfaction. There are of course many problems which require a mixed human sciences approach such as stress, fatigue and shift work.
Most of the American pioneers in this field were involved in either experimental psychology or engineering and it is for this reason that their typical occupational titles—human engineering and human factors—reflect a difference in emphasis (but not in core interests) from European ergonomics. This also explains why occupational hygiene, from its close relationship to medicine, particularly occupational medicine, is regarded in the United States as quite different from human factors or ergonomics. The difference in other parts of the world is less marked. Ergonomics concentrates on the human operator in action, occupational hygiene concentrates on the hazards to the human operator present in the ambient environment. Thus the central interest of the occupational hygienist is toxic hazards, which are outside the scope of the ergonomist. The occupational hygienist is concerned about effects on health, either long-term or short-term; the ergonomist is, of course, concerned about health but he or she is also concerned about other consequences, such as productivity, work design and workspace design. Safety and health are the generic issues which run through ergonomics, occupational hygiene, occupational health and occupational medicine. It is, therefore, not surprising to find that in a large institution of a research, design or production kind, these subjects are often grouped together. This makes possible an approach based on a team of experts in these separate subjects, each making a specialist contribution to the general problem of health, not only of the workers in the institution but also of those affected by its activities and products. By contrast, in institutions concerned with design or provision of services, the ergonomist might be closer to the engineers and other technologists.
It will be clear from this discussion that because ergonomics is interdisciplinary and still quite new there is an important problem of how it should best be fitted into an existing organization. It overlaps onto so many other fields because it is concerned with people and people are the basic and all-pervading resource of every organization. There are many ways in which it can be fitted in, depending on the history and objectives of the particular organization. The main criteria are that ergonomics objectives are understood and appreciated and that mechanisms for implementation of recommendations are built into the organization.
Aims of Ergonomics
It will be clear already that the benefits of ergonomics can appear in many different forms, in productivity and quality, in safety and health, in reliability, in job satisfaction and in personal development.
The reason for this breadth of scope is that its basic aim is efficiency in purposeful activity—efficiency in the widest sense of achieving the desired result without wasteful input, without error and without damage to the person involved or to others. It is not efficient to expend unnecessary energy or time because insufficient thought has been given to the design of the work, the workspace, the working environment and the working conditions. It is not efficient to achieve the desired result in spite of the situation design rather than with support from it.
The aim of ergonomics is to ensure that the working situation is in harmony with the activities of the worker. This aim is self-evidently valid but attaining it is far from easy for a variety of reasons. The human operator is flexible and adaptable and there is continuous learning, but there are quite large individual differences. Some differences, such as physical size and strength, are obvious, but others, such as cultural differences and differences in style and in level of skill, are less easy to identify.
In view of these complexities it might seem that the solution is to provide a flexible situation where the human operator can optimize a specifically appropriate way of doing things. Unfortunately such an approach is sometimes impracticable because the more efficient way is often not obvious, with the result that a worker can go on doing something the wrong way or in the wrong conditions for years.
Thus it is necessary to adopt a systematic approach: to start from a sound theory, to set measurable objectives and to check success against these objectives. The various possible objectives are considered below.
Safety and health
There can be no disagreement about the desirability of safety and health objectives. The difficulty stems from the fact that neither is directly measurable: their achievement is assessed by their absence rather than their presence. The data in question always pertain to departures from safety and health.
In the case of health, much of the evidence is long-term as it is based on populations rather than individuals. It is, therefore, necessary to maintain careful records over long periods and to adopt an epidemiological approach through which risk factors can be identified and measured. For example, what should be the maximum hours per day or per year required of a worker at a computer workstation? It depends on the design of the workstation, the kind of work and the kind of person (age, vision, abilities and so on). The effects on health can be diverse, from wrist problems to mental apathy, so it is necessary to carry out comprehensive studies covering quite large populations while simultaneously keeping track of differences within the populations.
Safety is more directly measurable in a negative sense in terms of kinds and frequencies of accidents and damage. There are problems in defining different kinds of accidents and identifying the often multiple causal factors and there is often a distant relationship between the kind of accident and the degree of harm, from none to fatality.
Nevertheless, an enormous body of evidence concerning safety and health has been accumulated over the past fifty years and consistencies have been discovered which can be related back to theory, to laws and standards and to principles operative in particular kinds of situations.
Productivity and efficiency
Productivity is usually defined in terms of output per unit of time, whereas efficiency incorporates other variables, particularly the ratio of output to input. Efficiency incorporates the cost of what is done in relation to achievement, and in human terms this requires the consideration of the penalties to the human operator.
In industrial situations, productivity is relatively easy to measure: the amount produced can be counted and the time taken to produce it is simple to record. Productivity data are often used in before/after comparisons of working methods, situations or conditions. It involves assumptions about equivalence of effort and other costs because it is based on the principle that the human operator will perform as well as is feasible in the circumstances. If the productivity is higher then the circumstances must be better. There is much to recommend this simple approach provided that it is used with due regard to the many possible complicating factors which can disguise what is really happening. The best safeguard is to try to make sure that nothing has changed between the before and after situations except the aspects being studied.
Efficiency is a more comprehensive but always a more difficult measure. It usually has to be specifically defined for a particular situation and in assessing the results of any studies the definition should be checked for its relevance and validity in terms of the conclusions being drawn. For example, is bicycling more efficient than walking? Bicycling is much more productive in terms of the distance that can be covered on a road in a given time, and it is more efficient in terms of energy expenditure per unit of distance or, for indoor exercise, because the apparatus required is cheaper and simpler. On the other hand, the purpose of the exercise might be energy expenditure for health reasons or to climb a mountain over difficult terrain; in these circumstances walking will be more efficient. Thus, an efficiency measure has meaning only in a well-defined context.
Reliability and quality
As explained above, reliability rather than productivity becomes the key measure in high technology systems (for instance, transport aircraft, oil refining and power generation). The controllers of such systems monitor performance and make their contribution to productivity and to safety by making tuning adjustments to ensure that the automatic machines stay on line and function within limits. All these systems are in their safest states either when they are quiescent or when they are functioning steadily within the designed performance envelope. They become more dangerous when moving or being moved between equilibrium states, for example, when an aircraft is taking off or a process system is being shut down. High reliability is the key characteristic not only for safety reasons but also because unplanned shut-down or stoppage is extremely expensive. Reliability is straightforward to measure after performance but is extremely difficult to predict except by reference to the past performance of similar systems. When or if something goes wrong human error is invariably a contributing cause, but it is not necessarily an error on the part of the controller: human errors can originate at the design stage and during setting up and maintenance. It is now accepted that such complex high-technology systems require a considerable and continuous ergonomics input from design to the assessment of any failures that occur.
Quality is related to reliability but is very difficult if not impossible to measure. Traditionally, in batch and flow production systems, quality has been checked by inspection after output, but the current established principle is to combine production and quality maintenance. Thus each operator has parallel responsibility as an inspector. This usually proves to be more effective, but it may mean abandoning work incentives based simply on rate of production. In ergonomic terms it makes sense to treat the operator as a responsible person rather than as a kind of robot programmed for repetitive performance.
Job satisfaction and personal development
From the principle that the worker or human operator should be recognized as a person and not a robot it follows that consideration should be given to responsibilities, attitudes, beliefs and values. This is not easy because there are many variables, mostly detectable but not quantifiable, and there are large individual and cultural differences. Nevertheless a great deal of effort now goes into the design and management of work with the aim of ensuring that the situation is as satisfactory as is reasonably practicable from the operator’s viewpoint. Some measurement is possible by using survey techniques and some principles are available based on such working features as autonomy and empowerment.
Even accepting that these efforts take time and cost money, there can still be considerable dividends from listening to the suggestions, opinions and attitudes of the people actually doing the work. Their approach may not be the same as that of the external work designer and not the same as the assumptions made by the work designer or manager. These differences of view are important and can provide a refreshing change in strategy on the part of everyone involved.
It is well established that the human being is a continuous learner or can be, given the appropriate conditions. The key condition is to provide feedback about past and present performance which can be used to improve future performance. Moreover, such feedback itself acts as an incentive to performance. Thus everyone gains, the performer and those responsible in a wider sense for the performance. It follows that there is much to be gained from performance improvement, including self-development. The principle that personal development should be an aspect of the application of ergonomics requires greater designer and manager skills but, if it can be applied successfully, can improve all the aspects of human performance discussed above.
Successful application of ergonomics often follows from doing no more than developing the appropriate attitude or point of view. The people involved are inevitably the central factor in any human effort and the systematic consideration of their advantages, limitations, needs and aspirations is inherently important.
Ergonomics is the systematic study of people at work with the objective of improving the work situation, the working conditions and the tasks performed. The emphasis is on acquiring relevant and reliable evidence on which to base recommendation for changes in specific situations and on developing more general theories, concepts, guidelines and procedures which will contribute to the continually developing expertise available from ergonomics.
It is difficult to speak of work analysis without putting it in the perspective of recent changes in the industrial world, because the nature of activities and the conditions in which they are carried out have undergone considerable evolution in recent years. The factors giving rise to these changes have been numerous, but there are two whose impact has proved crucial. On the one hand, technological progress with its ever-quickening pace and the upheavals brought about by information technologies have revolutionized jobs (De Keyser 1986). On the other hand, the uncertainty of the economic market has required more flexibility in personnel management and work organization. If the workers have gained a wider view of the production process that is less routine-oriented and undoubtedly more systematic, they have at the same time lost exclusive links with an environment, a team, a production tool. It is difficult to view these changes with serenity, but we have to face the fact that a new industrial landscape has been created, sometimes more enriching for those workers who can find their place in it, but also filled with pitfalls and worries for those who are marginalized or excluded. However, one idea is being taken up in firms and has been confirmed by pilot experiments in many countries: it should be possible to guide changes and soften their adverse effects with the use of relevant analyses and by using all resources for negotiation between the different work actors. It is within this context that we must place work analyses today—as tools allowing us to describe tasks and activities better in order to guide interventions of different kinds, such as training, the setting up of new organizational modes or the design of tools and work systems. We speak of analyses, and not just one analysis, since there exist a large number of them, depending on the theoretical and cultural contexts in which they are developed, the particular goals they pursue, the evidence they collect, or the analyser’s concern for either specificity or generality. In this article, we will limit ourselves to presenting a few characteristics of work analyses, and emphasizing the importance of collective work. Our conclusions will highlight other paths that the limits of this text prevent us from pursuing in greater depth.
Some Characteristics of Work Analyses
If the primary goal of any work analysis is to describe what the operator does, or should do, placing it more precisely into its context has often seemed indispensable to researchers. They mention, according to their own views, but in a broadly similar manner, the concepts of context, situation, environment, work domain, work world or work environment. The problem lies less in the nuances between these terms than in the selection of variables that need to be described in order to give them a useful meaning. Indeed, the world is vast and industry is complex, and the characteristics that could be referred to are innumerable. Two tendencies can be noted among authors in the field. The first one sees the description of the context as a means of capturing the reader’s interest and providing him or her with an adequate semantic framework. The second has a different theoretical perspective: it attempts to embrace both context and activity, describing only those elements of the context that are capable of influencing the behaviour of operators.
The semantic framework
Context has evocative power. It is enough, for an informed reader, to read about an operator in a control room engaged in a continuous process to call up a picture of work through commands and surveillance at a distance, where the tasks of detection, diagnosis and regulation predominate. What variables need to be described in order to create a sufficiently meaningful context? It all depends on the reader. Nonetheless, there is a consensus in the literature on a few key variables. The nature of the economic sector, the type of production or service, the size and the geographical location of the site are useful.
The production processes, the tools or machines and their level of automation allow certain constraints and certain necessary qualifications to be guessed at. The structure of the personnel, together with age and level of qualification and experience are crucial data whenever the analysis concerns aspects of training or of organizational flexibility. The organization of work established depends more on the firm’s philosophy than on technology. Its description includes, notably, work schedules, the degree of centralization of decisions and the types of control exercised over the workers. Other elements may be added in different cases. They are linked to the firm’s history and culture, its economic situation, work conditions and any restructuring, mergers and investments. There exist at least as many systems of classification as there are authors, and there are numerous descriptive lists in circulation. In France, a special effort has been made to generalize simple descriptive methods, notably allowing for the ranking of certain factors according to whether or not they are satisfactory for the operator (RNUR 1976; Guelaud et al. 1977).
The description of relevant factors regarding the activity
The taxonomy of complex systems described by Rasmussen, Pejtersen and Schmidts (1990) represents one of the most ambitious attempts to cover at the same time the context and its influence on the operator. Its main idea is to integrate, in a systematic fashion, the different elements of which it is composed and to bring out the degrees of freedom and the constraints within which individual strategies can be developed. Its exhaustive aim makes it difficult to manipulate, but the use of multiple modes of representation, including graphs, to illustrate the constraints has a heuristic value that is bound to be attractive to many readers. Other approaches are more targeted. What the authors seek is the selection of factors that can influence a precise activity. Hence, with an interest in the control of processes in a changing environment, Brehmer (1990) proposes a series of temporal characteristics of the context which affect the control and anticipation of the operator (see figure 1). This author’s typology has been developed from “micro-worlds”, computerized simulations of dynamic situations, but the author himself, along with many others since, used it for the continuous-process industry (Van Daele 1992). For certain activities, the influence of the environment is well known, and the selection of factors is not too difficult. Thus, if we are interested in heart rate in the work environment, we often limit ourselves to describing the air temperatures, the physical constraints of the task or the age and training of the subject—even though we know that by doing so we perhaps leave out relevant elements. For others, the choice is more difficult. Studies on human error, for example, show that the factors capable of producing them are numerous (Reason 1989). Sometimes, when theoretical knowledge is insufficient, only statistical processing, combining context and activity analysis, allows us to bring out the relevant contextual factors (Fadier 1990).
Figure 1. The criteria and sub-criteria of the taxonomy of micro-worlds proposed by Brehmer (1990)
The Task or the Activity?
The task is defined by its objectives, its constraints and the means it requires for achievement. A function within the firm is generally characterized by a set of tasks. The realized task differs from the prescribed task scheduled by the firm for a large number of reasons: the strategies of operators vary within and among individuals, the environment fluctuates and random events require responses that are often outside the prescribed framework. Finally, the task is not always scheduled with correct knowledge of its conditions of execution, hence the need for adaptations in real time. But even if the task is updated during the activity, sometimes to the point of being transformed, it still remains the central reference.
Questionnaires, inventories and taxonomies of tasks are numerous, especially in the English-language literature—the reader will find excellent reviews in Fleishman and Quaintance (1984) and in Greuter and Algera (1989). Certain of these instruments are merely lists of elements—for example, the action verbs to illustrate tasks—that are checked off according to the function studied. Others have adopted a hierarchical principle, characterizing a task as interlocking elements, ordered from the global to the particular. These methods are standardized and can be applied to a large number of functions; they are simple to use, and the analytical stage is much shortened. But where it is a question of defining specific work, they are too static and too general to be useful.
Next, there are those instruments requiring more skill on the part of the researcher; since the elements of analysis are not predefined, it is up to the researcher to characterize them. The already outdated critical incident technique of Flanagan (1954), where the observer describes a function by reference to its difficulties and identifies the incidents which the individual will have to face, belongs to this group.
It is also the path adopted by cognitive task analysis (Roth and Woods 1988). This technique aims to bring to light the cognitive requirements of a job. One way to do this is to break the job down into goals, constraints and means. Figure 2 shows how the task of an anaesthetist, characterized first by a very global goal of patient survival, can be broken down into a series of sub-goals, which can themselves be classified as actions and means to be employed. More than 100 hours of observation in the operating theatre and subsequent interviews with anaesthetists were necessary to obtain this synoptic “photograph” of the requirements of the function. This technique, although quite laborious, is nevertheless useful in ergonomics in determining whether all the goals of a task are provided with the means of attaining them. It also allows for an understanding of the complexity of a task (its particular difficulties and conflicting goals, for example) and facilitates the interpretation of certain human errors. But it suffers, as do other methods, from the absence of a descriptive language (Grant and Mayes 1991). Moreover, it does not permit hypotheses to be formulated as to the nature of the cognitive processes brought into play to attain the goals in question.
Figure 2. Cognitive analysis of the task: general anaesthesia
Other approaches have analysed the cognitive processes associated with given tasks by drawing up hypotheses as to the information processing necessary to accomplish them. A frequently employed cognitive model of this kind is Rasmussen’s (1986), which provides, according to the nature of the task and its familiarity for the subject, three possible levels of activity—based either on skill-based habits and reflexes, on acquired rule-based procedures or on knowledge-based procedures. But other models or theories that reached the height of their popularity during the 1970s remain in use. Hence, the theory of optimal control, which considers man as a controller of discrepancies between assigned and observed goals, is sometimes still applied to cognitive processes. And modelling by means of networks of interconnected tasks and flow charts continues to inspire the authors of cognitive task analysis; figure 3 provides a simplified description of the behavioural sequences in an energy-control task, constructing a hypothesis about certain mental operations. All these attempts reflect the concern of researchers to bring together in the same description not only elements of the context, but also the task itself and the cognitive processes that underlie it—and to reflect the dynamic character of work as well.
Figure 3. Simplified description of the determinants of a behaviour sequence in an energy control taks: case of unacceptable consumption of energy
Since the arrival of the scientific organization of work, the concept of the prescribed task has been adversely criticized because it has been viewed as involving the imposition on workers of tasks that are not only designed without consulting their needs, but are often accompanied by a specific performance time, a restriction not welcomed by many workers. Even if the imposition aspect has become rather more flexible today and even if the workers contribute more often to the design of tasks, an assigned time for tasks remains necessary for schedule planning and remains an essential component of work organization. The quantification of time should not always be perceived in a negative manner. It constitutes a valuable indicator of workload. A simple but common method of measuring the time pressure exerted on a worker consists of determining the quotient of the time necessary for the execution of a task divided by the available time. The closer this quotient is to unity, the greater the pressure (Wickens 1992). Moreover, quantification can be used in flexible but appropriate personnel management. Let us take the case of nurses where the technique of predictive analysis of tasks has been generalized, for example, in the Canadian regulation Planning of Required Nursing (PRN 80) (Kepenne 1984) or one of its European variants. Thanks to such task lists, accompanied by their mean time of execution, one can, each morning, taking into account the number of patients and their medical conditions, establish a care schedule and a distribution of personnel. Far from being a constraint, PRN 80 has, in a number of hospitals, demonstrated that a shortage of nursing personnel exists, since the technique allows a difference to be established (see figure 4) between the desired and the observed, that is, between the number of staff necessary and the number available, and even between the tasks planned and the tasks carried out. The times calculated are only averages, and the fluctuations in the situation do not always make them applicable, but this negative aspect is minimized by a flexible organization that accepts adjustments and allows the personnel to participate in effecting those adjustments.
Figure 4. Discrepancies between the numbers of personnel present and required on the basis of PRN80
The activity, the evidence and the performance
An activity is defined as the set of behaviours and resources used by the operator so that work occurs—that is to say, the transformation or production of goods or the rendering of a service. This activity can be understood through observation in different ways. Faverge (1972) has described four forms of analysis. The first is an analysis in terms of gestures and postures, where the observer locates, within the visible activity of the operator, classes of behaviour that are recognizable and repeated during work. These activities are often coupled with a precise response: for example, the heart rate, which allows us to assess the physical load associated with each activity. The second form of analysis is in terms of information uptake. What is discovered, through direct observation—or with the aid of cameras or recorders of eye movements—is the set of signals picked up by the operator in the information field surrounding him or her. This analysis is particularly useful in cognitive ergonomics in trying to better understand the information processing carried out by the operator. The third type of analysis is in terms of regulation. The idea is to identify the adjustments of activity carried out by the operator in order to deal with either fluctuations in the environment or changes in his own condition. There we find the direct intervention of context within the analysis. One of the most frequently cited research projects in this area is that of Sperandio (1972). This author studied the activity of air traffic controllers and identified important strategy changes during an increase in air traffic. He interpreted them as an attempt to simplify the activity by aiming to maintain an acceptable load level, while at the same time continuing to meet the requirements of the task. The fourth is an analysis in terms of thought processes. This type of analysis has been widely used in the ergonomics of highly automated posts. Indeed, the design of computerized aids, and notably intelligent aids for the operator, requires a thorough understanding of the way in which the operator reasons in order to solve certain problems. The reasoning involved in scheduling, anticipation and diagnosis has been the subject of analyses, an example of which can be found in figure 5. However, the evidence of mental activity can only be inferred. Apart from certain observable aspects of behaviour, such as eye movements and problem-solving time, most of these analyses resort to verbal response. Particular emphasis has been placed, in recent years, on the knowledge necessary to accomplish certain activities, with researchers trying not to postulate them at the outset but to make them apparent through the analysis itself.
Figure 5. Analysis of mental activity. Strategies in the control of processes with long response times: need for computerized support in diagnosis
Such efforts have brought to light the fact that almost identical performances can be obtained with very different levels of knowledge, as long as operators are aware of their limits and apply strategies adapted to their capabilities. Hence, in our study of the start-up of a thermoelectric plant (De Keyser and Housiaux 1989), the start-ups were carried out by both engineers and operators. The theoretical and procedural knowledge that these two groups possessed, which had been elicited by means of interviews and questionnaires, were very different. The operators in particular sometimes had an erroneous understanding of the variables in the functional links of the process. In spite of this, the performances of the two groups were very close. But the operators took into account more variables in order to verify the control of the start-up and undertook more frequent verifications. Such results were also obtained by Amalberti (1991), who mentioned the existence of metaknowledges allowing experts to manage their own resources.
What evidence of activity is appropriate to elicit? Its nature, as we have seen, depends closely on the form of analysis planned. Its form varies according to the degree of methodological care exercised by the observer. Provoked evidence is distinguished from spontaneous evidence, and concomitant from subsequent evidence. Generally speaking, when the nature of the work allows, concomitant and spontaneous evidence are to be preferred. They are free of various drawbacks such as unreliability of memory, observer interference, the effect of rationalizing reconstruction on the part of the subject, and so forth. To illustrate these distinctions, we will take the example of verbalizations. Spontaneous verbalizations are verbal exchanges, or monologues expressed spontaneously without being requested by the observer; provoked verbalizations are those made at the specific request of the observer, such as the request made to the subject to “think aloud”, which is well known in the cognitive literature. Both types can be done in real time, during work, and are thus concomitant.
They can also be subsequent, as in interviews, or subjects’ verbalizations when they view videotapes of their work. As for the validity of the verbalizations, the reader should not ignore the doubt raised in this regard by the controversy between Nisbett and De Camp Wilson (1977) and White (1988) and the precautions suggested by numerous authors aware of their importance in the study of mental activity in view of the methodological difficulties encountered (Ericson and Simon 1984; Savoyant and Leplat 1983; Caverni 1988; Bainbridge 1986).
The organization of this evidence, its processing and its formalization require descriptive languages and sometimes analyses which go beyond field observation. Those mental activities which are inferred from the evidence, for example, remain hypothetical. Today they are often described using languages derived from artificial intelligence, making use of representations in terms of schemes, production rules and connecting networks. Moreover, the use of computerized simulations—of micro-worlds—to pinpoint certain mental activities has become widespread, even though the validity of the results obtained from such computerized simulations, in view of the complexity of the industrial world, is subject to debate. Finally, we must mention the cognitive modellings of certain mental activities extracted from the field. Among the best known are the diagnosis of the operator of a nuclear power plant, carried out in ISPRA (Decortis and Cacciabue 1990), and the planning of the combat pilot perfected in Centre d’études et de recherches de médecine aérospatiale (CERMA) (Amalberti et al. 1989).
Measurement of the discrepancies between the performance of these models and that of real, living operators is a fruitful field in activity analysis. Performance is the outcome of the activity, the final response given by the subject to the requirements of the task. It is expressed at the level of production: productivity, quality, error, incident, accident—and even, at a more global level, absenteeism or turnover. But it must also be identified at the individual level: the subjective expression of satisfaction, stress, fatigue or workload, and many physiological responses are also performance indicators. Only the entire set of data permits interpretation of the activity—that is to say, judging whether or not it furthers the desired goals, while remaining within human limits. There exists a set of norms which, up to a certain point, guide the observer. But these norms are not situated—they do not take into account the context, its fluctuations and the condition of the worker. This is why in design ergonomics, even when rules, norms and models exist, designers are advised to test the product using prototypes as early as possible and to evaluate the users’ activity and performance.
Individual or Collective Work?
While in the vast majority of cases, work is a collective act, most work analyses focus on tasks or individual activities. Nonetheless the fact is that technological evolution, just like work organization, today emphasizes distributed work, whether it be between workers and machines or simply within a group. What paths have been explored by authors so as to take this distribution into account (Rasmussen, Pejtersen and Schmidts 1990)? They focus on three aspects: structure, the nature of exchanges and structural lability.
Whether we view structure as elements of the analysis of people, or of services, or even of different branches of a firm working in a network, the description of the links that unite them remains a problem. We are very familiar with the organigrams within firms that indicate the structure of authority and whose various forms reflect the organizational philosophy of the firm—very hierarchically organized for a Taylor-like structure, or flattened like a rake, even matrix-like, for a more flexible structure. Other descriptions of distributed activities are possible: an example is given in figure 6. More recently, the need for firms to represent their information exchanges at a global level has led to a rethinking of information systems. Thanks to certain descriptive languages—for example, design schemas, or entity-relations-attribute matrixes—the structure of relations at the collective level can today be described in a very abstract manner and can serve as a springboard for the creation of computerized management systems.
Figure 6. Integrated life cycle design
The nature of exchanges
Simply having a description of the links uniting the entities says little about the content itself of the exchanges; of course the nature of the relation can be specified—movement from place to place, information transfers, hierarchical dependence, and so on—but this is often quite inadequate. The analysis of communications within teams has become a favoured means of capturing the very nature of collective work, encompassing subjects mentioned, creation of a common language in a team, modification of communications when circumstances are critical, and so forth (Tardieu, Nanci and Pascot 1985; Rolland 1986; Navarro 1990; Van Daele 1992; Lacoste 1983; Moray, Sanderson and Vincente 1989). Knowledge of these interactions is particularly useful for the creation of computer tools, notably decision-making aids for understanding errors. The different stages and the methodological difficulties linked to the use of this evidence have been well described by Falzon (1991).
It is the work on activities rather than on tasks that has opened up the field of structural lability—that is to say, of the constant reconfigurations of collective work under the influence of contextual factors. Studies such as those of Rogalski (1991), who over a long period analysed the collective activities dealing with forest fires in France, and Bourdon and Weill Fassina (1994), who studied the organizational structure set up to deal with railway accidents, are both very informative. They clearly show how the context moulds the structure of exchanges, the number and type of actors involved, the nature of the communications and the number of parameters essential to the work. The more this context fluctuates, the further the fixed descriptions of the task are removed from reality. Knowledge of this lability, and a better understanding of the phenomena that take place within it, are essential in planning for the unpredictable and in order to provide better training for those involved in collective work in a crisis.
The various phases of the work analysis that have been described are an iterative part of any human factors design cycle (see figure 6). In this design of any technical object, whether a tool, a workstation or a factory, in which human factors are a consideration, certain information is needed in time. In general, the beginning of the design cycle is characterized by a need for data involving environmental constraints, the types of jobs that are to be carried out, and the various characteristics of the users. This initial information allows the objects specifications to be drawn up so as to take into account work requirements. But this is, in some sense, only a coarse model compared to the real work situation. This explains why models and prototypes are necessary that, from their inception, allow not the jobs themselves, but the activities of the future users to be evaluated. Consequently, while the design of the images on a monitor in a control room can be based on a thorough cognitive analysis of the job to be done, only a data-based analysis of the activity will allow an accurate determination of whether the prototype will actually be of use in the actual work situation (Van Daele 1988). Once the finished technical object is put into operation, greater emphasis is put on the performance of the users and on dysfunctional situations, such as accidents or human error. The gathering of this type of information allows the final corrections to be made that will increase the reliability and usability of the completed object. Both the nuclear industry and the aeronautics industry serve as example: operational feedback involves reporting every incident that occurs. In this way, the design loop comes full circle.
Standardization in the field of ergonomics has a relatively short history. It started in the beginning of the 1970s when the first committees were founded on the national level (e.g., in Germany within the standardization institute DIN), and it continued on an international level after the foundation of the ISO (International Organization for Standardization) TC (Technical Committee) 159 “Ergonomics”, in 1975. In the meantime ergonomics standardization takes place on regional levels as well, for example, on the European level within the CEN (Commission européenne de normalisation), which established its TC 122 “Ergonomics” in 1987. The existence of the latter committee underscores the fact that one of the important reasons for establishing committees for the standardization of ergonomics knowledge and principles can be found in legal (and quasi-legal) regulations, especially with respect to safety and health, which require the application of ergonomics principles and findings in the design of products and work systems. National laws requiring the application of well-established ergonomics findings were the reason for the establishment of the German ergonomics committee in 1970, and European Directives, especially the Machinery Directive (relating to safety standards), were responsible for establishing an ergonomics committee on the European level. Since legal regulations usually are not, cannot and should not be very specific, the task of specifying which ergonomics principles and findings should be applied was given to or taken up by ergonomics standardization committees. Especially on the European level, it can be recognized that ergonomics standardization can contribute to the task of providing for broad and comparable conditions of machinery safety, thus removing barriers to the free trade of machinery within the continent itself.
Ergonomics standardization thus started with a strong protective, although preventive, perspective, with ergonomics standards being developed with the aim of protecting workers against adverse effects at different levels of health protection. Ergonomics standards were thus prepared with the following intentions in view:
International standardization, which was not so closely coupled to legislation, on the other hand, always also tried to open a perspective in the direction of producing standards which would go beyond the prevention of and protection against adverse effects (e.g., by specifying minimal/maximal values) and instead proactively provide for optimal working conditions to promote the well-being and personal development of the worker, as well as the effectiveness, efficiency, reliability and productivity of the work system.
This is a point where it becomes evident that ergonomics, and especially ergonomics standardization, has very distinct social and political dimensions. Whereas the protective approach with respect to safety and health is generally accepted and agreed upon among the parties involved (employers, unions, administration and ergonomics experts) for all levels of standardization, the proactive approach is not equally accepted by all parties in the same way. This might be due to the fact that, especially where legislation requires the application of ergonomics principles (and thus either explicitly or implicitly the application of ergonomics standards), some parties feel that such standards might limit their freedom of action or negotiation. Since international standards are less compelling (transferring them into the body of national standards is at the discretion of the national standardization committees) the proactive approach has been developed furthest at the international level of ergonomics standardization.
The fact that certain regulations would indeed restrict the discretion of those to whom they applied served to discourage standardization in certain areas, for example in connection with the European Directives under Article 118a of the Single European Act, relating to safety and health in the use and operation of machinery at the workplace, and in the design of work systems and workplace design. On the other hand, under the Directives issued under Article 100a, relating to safety and health in the design of machinery with regard to the free trade of this machinery within the European Union (EU), European ergonomics standardization is mandated by the European Commission.
From an ergonomics point of view, however, it is difficult to understand why ergonomics in the design of machinery should be different from that in the use and operation of machinery within a work system. It is thus to be hoped that the distinction will be given up in the future, since it seems to be more detrimental than beneficial to the development of a consistent body of ergonomics standards.
Types of Ergonomics Standards
The first international ergonomics standard to have been developed (based on a German DIN national standard) is ISO 6385, “Ergonomic principles in the design of work systems”, published in 1981. It is the basic standard of the ergonomics standards series and set the stage for the standards which followed by defining the basic concepts and stating the general principles of the ergonomic design of work systems, including tasks, tools, machinery, workstations, work space, work environment and work organization. This international standard, which is now undergoing revision, is a guideline standard, and as such provides guidelines to be followed. It does not, however, provide technical or physical specifications which have to be met. These can be found in a different type of standards, that is, specification standards, for example, those on anthropometry or thermal conditions. Both types of standards fulfil different functions. While guideline standards intend to show their users “what to do and how to do it” and indicate those principles that must or should be observed, for example, with respect to mental workload, specification standards provide users with detailed information about safety distances or measurement procedures, for example, that have to be met and where compliance with these prescriptions can be tested by specified procedures. This is not always possible with guideline standards, although despite their relative lack of specificity it can usually be demonstrated when and where guidelines have been violated. A subset of specification standards are “database” standards, which provide the user with relevant ergonomics data, for example, body dimensions.
CEN standards are classified as A-, B- and C-type standards, depending on their scope and field of application. A-type standards are general, basic standards which apply to all kinds of applications, B-type standards are specific for an area of application (which means that most of the ergonomics standards within the CEN will be of this type), and C-type standards are specific for a certain kind of machinery, for example, hand-held drilling machines.
Ergonomics standards, like other standards, are produced in the appropriate technical committees (TCs), their subcommittees (SCs) or working groups (WGs). For the ISO this is TC 159, for CEN it is TC 122, and on the national level, the respective national committees. Besides the ergonomics committees, ergonomics is also dealt with in TCs working on machine safety (e.g., CEN TC 114 and ISO TC 199) with which liaison and close cooperation is maintained. Liaisons are also established with other committees for which ergonomics might be of relevance. Responsibility for ergonomics standards, however, is reserved to the ergonomics committees themselves.
A number of other organizations are engaged in the production of ergonomics standards, such as the IEC (International Electrotechnical Commission); CENELEC, or the respective national committees in the electrotechnical field; CCITT (Comité consultative international des organisations téléphoniques et télégraphiques) or ETSI (European Telecommunication Standards Institute) in the field of telecommunications; ECMA (European Computer Manufacturers Association) in the field of computer systems; and CAMAC (Computer Assisted Measurement and Control Association) in the field of new technologies in manufacturing, to name only a few. With some of these the ergonomics committees do have liaisons in order to avoid duplication of work or inconsistent specifications; with some organizations (e.g., the IEC) even joint technical committees are established for cooperation in areas of mutual interest. With other committees, however, there is no coordination or cooperation at all. The main purpose of these committees is to produce (ergonomics) standards that are specific to their field of activity. Since the number of such organizations at the different levels is rather large, it becomes quite complicated (if not impossible) to carry out a complete overview of ergonomics standardization. The present review will therefore be restricted to ergonomics standardization in the international and European ergonomics committees.
Structure of Standardization Committees
Ergonomics standardization committees are quite similar to one another in structure. Usually one TC within a standardization organization is responsible for ergonomics. This committee (e.g., ISO TC 159) mainly has to do with decisions about what should be standardized (e.g., work items) and how to organize and coordinate the standardization within the committee, but usually no standards are prepared at this level. Below the TC level are other committees. For example, the ISO has subcommittees (SCs), which are responsible for a defined field of standardization: SC 1 for general ergonomic guiding principles, SC 3 for anthropometry and biomechanics, SC 4 for human-system interaction and SC 5 for the physical work environment. CEN TC 122 has working groups (WGs) below the TC level which are so constituted as to deal with specified fields within ergonomics standardization. SCs within ISO TC 159 operate as steering committees for their field of responsibility and do the first voting, but usually they do not also prepare standards. This is done in their WGs, which are composed of experts nominated by their national committees, whereas SC and TC meetings are attended by national delegations representing national points of view. Within the CEN, duties are not sharply distinguished at the WG level; WGs operate both as steering and production committees, although a good deal of work is accomplished in ad hoc groups, which are composed of members of the WG (nominated by their national committees) and established to prepare the drafts for a standard. WGs within an ISO SC are established to do the practical standardization work, that is, prepare drafts, work on comments, identify needs for standardization, and prepare proposals to the SC and TC, which will then take the appropriate decisions or actions.
Preparation of Ergonomics Standards
The preparation of ergonomics standards has changed quite markedly within the last years in view of the stronger emphasis now being placed on European and other international developments. In the beginning, national standards, which had been prepared by experts from one country in their national committee and agreed upon by the interested parties among the general public of that country in a specified voting procedure, were transferred as input to the responsible SC and WG of ISO TC 159, after a formal vote had been taken at the TC level that such an international standard should be prepared. The working group, composed of ergonomics experts (and experts from politically interested parties) from all participating member bodies (i.e., the national standardization organizations) of TC 159 who were willing to cooperate in this work project, would then work on any inputs and prepare a working draft (WD). After this draft proposal is agreed upon in the WG, it becomes a committee draft (CD), which is distributed to the member bodies of the SC for approval and comments. If the draft receives substantial support from the SC member bodies (i.e., if at least two-thirds vote in favour) and after comments by the national committees have been incorporated by the WG in the improved version, a Draft International Standard (DIS) is submitted for voting to all members of TC 159. If substantial support, at this step from the member bodies of the TC, is achieved (and perhaps after incorporating editorial changes), this version will then be published as an International Standard (IS) by the ISO. Voting of the member bodies at the TC and SC level is based on voting at the national level, and comments can be supplied through the member bodies by experts or interested parties in each country. The procedure is roughly equivalent in CEN TC 122, with the exception that there are no SCs below the TC level and that voting takes part with weighted votes (according to the size of the country) whereas within the ISO the rule is one country, one vote. If a draft fails at any step, and unless the WG decides that an agreeable revision cannot be achieved, it has to be revised and then has to pass through the voting procedure again.
International standards are then transferred into national standards if the national committees vote accordingly. By contrast, European Standards (ENs) have to be transferred into national standards by the CEN members and conflicting national standards have to be withdrawn. That means that harmonized ENs will be effective in all CEN countries (and, due to their influence on trade, will be relevant to manufacturers in all other countries who intend to sell goods to a customer in a CEN country).
In order to avoid conflicting standards and duplication of work and to allow non-CEN members to take part in developments in the CEN, a cooperative agreement between the ISO and the CEN has been achieved (the so-called Vienna Agreement) which regulates the formalities and provides for a so-called parallel voting procedure, which allows the same drafts to be voted upon in the CEN and the ISO in parallel, if the responsible committees agree to do so. Among the ergonomics committees the tendency is quite clear: avoid duplication of work (manpower and financial resources are too limited), avoid conflicting specifications, and try to achieve a consistent body of ergonomics standards based on a division of labour. Whereas CEN TC 122 is bound by the decisions of the EU administration and gets mandated work items to stipulate the specifications of European directives, ISO TC 159 is free to standardize whatever it thinks necessary or appropriate in the field of ergonomics. This has led to shifts in the emphasis of both committees, with the CEN concentrating on machinery and safety-related topics and the ISO concentrating on areas where broader market interests than Europe are concerned (e.g., work with VDUs and control-room design for process and related industries); on areas where the operation of machinery is concerned, as in work system design; and on such areas as work environment and work organization as well. The intention, however, is to transfer work results from the CEN to the ISO, and vice versa, in order to build up a body of consistent ergonomics standards which in fact are effective all over the world.
The formal procedure of producing standards is still the same today. But since the emphasis has shifted more and more to the international or the European level, more and more activities are being transferred to these committees. Drafts are now usually worked out directly in these committees and are no longer based on existing national standards. After the decision has been made that a standard should be developed, work directly starts at one of these supranational levels, based on whatever input there may be available, sometimes starting from zero. This changes the role of the national ergonomics committees quite dramatically. While heretofore they formally developed their own national standards according to their national rules, they now have the task of observing and influencing standardization on the supranational levels—via the experts who work out the standards or via comments made at the different steps of voting (within the CEN, a national standardization project will be halted if a comparable project is being simultaneously worked on at the CEN level). This makes the task still more complicated, since this influence can only be exerted indirectly and since the preparation of ergonomics standards is not just a matter of pure science but a matter of bargaining, consensus and agreement (not least due to the political implications which the standard might have). This, of course, is one of the reasons why the process of producing an international or European ergonomics standard usually takes several years and why ergonomics standards cannot reflect the latest state of the art in ergonomics. International ergonomics standards thus have to be examined every five years, and, if necessary, undergo revision.
Fields of Ergonomics Standardization
International ergonomics standardization started with guidelines on the general principles of ergonomics in the design of work systems; they were laid down in ISO 6385, which is now under revision in order to incorporate new developments. The CEN has produced a similar basic standard (EN 614, Part 1, 1994)—this is oriented more to machinery and safety—and is preparing a standard with guidelines on task design as a second part of this basic standard. The CEN thus emphasizes the importance of operator tasks in the design of machinery or work systems, for which appropriate tools or machinery have to be designed.
Another area where concepts and guidelines have been laid down in standards is the field of mental workload. ISO 10075, Part 1, defines terms and concepts (e.g., fatigue, monotony, reduced vigilance), and Part 2 (at the stage of a DIS in the latter half of the 1990s) provides guidelines for the design of work systems with respect to mental workload in order to avoid impairments.
SC 3 of ISO TC 159 and WG 1 of CEN TC 122 produce standards on anthropometry and biomechanics, covering, among other topics, methods of anthropometric measurements, body dimensions, safety distances and access dimensions, the evaluation of working postures and the design of workplaces in relation to machinery, recommended limits of physical strength and problems of manual handling.
SC 4 of ISO 159 shows how technological and social changes affect ergonomics standardization and the programme of such a subcommittee. SC 4 started as “Signals and Controls” by standardizing principles for displaying information and designing control actuators, with one of its work items being the visual display unit (VDU), used for office tasks. It soon became apparent, however, that standardizing the ergonomics of VDUs would not be sufficient, and that standardization “around” this workstation—in the sense of a work system—was required, covering areas such as hardware (e.g., the VDU itself, including displays, keyboards, non-keyboard input devices, workstations), work environment (e.g., lighting), work organization (e.g., task requirements), and software (e.g., dialogue principles, menu and direct manipulation dialogues). This led to a multipart standard (ISO 9241) covering “ergonomic requirements for office work with VDUs” with at the moment 17 parts, 3 of which have reached the status of an IS already. This standard will be transferred to the CEN (as EN 29241) which will specify requirements for the VDU directive (90/270 EEC) of the EU—although this is a directive under article 118a of the Single European Act. This series of standards provides guidelines as well as specifications, depending on the subject of the given part of the standard, and introduces a new concept of standardization, the user performance approach, which might help to solve some of the problems in ergonomics standardization. It is described more fully in the chapter Visual Display Units .
The user performance approach is based on the idea that the aim of standardization is to prevent impairment and to provide for optimal working conditions for the operator, but not to establish technical specification per se. Specification is thus regarded only as a means to the end of unimpaired, optimal user performance. The important thing is to achieve this unimpaired performance of the operator, regardless of whether a certain physical specification is met. This requires that the unimpaired operator performance which has to be achieved, for example, reading performance on a VDU, must be specified in the first place, and second, that technical specifications be developed which will enable the desired performance to be achieved, based on the available evidence. The manufacturer is then free to follow these technical specifications, which will ensure that the product complies with the ergonomics requirements. Or he may demonstrate, by comparison with a product that is known to fulfil the requirements (either by compliance with the technical specifications of the standard or by proven performance), that with the new product the performance requirements are equally or better fulfilled than with the reference product, with or without compliance to the technical specifications of the standard. A test procedure which has to be followed for demonstrating conformance with the user performance requirements of the standard is specified in the standard.
This approach helps to overcome two problems. Standards, by virtue of their specifications, which are based on the state of the art (and technology) at the time of preparation of the standard, can restrict new developments. Specifications that are based on a certain technology (e.g., cathode-ray tubes) may be inappropriate for other technologies. Independently of technology, however, the user of a display device (for instance) should be able to read and understand the information displayed effectively and efficiently without any impairments, irrespective of whatever technique may be used. Performance in this case must, however, not be restricted to the pure output (as measured in terms of speed or accuracy) but must include considerations of comfort and effort as well.
The second problem that can be dealt with by this approach is the problem of interactions between conditions. Physical specification usually is unidimensional, leaving other conditions out of consideration. In the case of interactive effects, however, this can be misleading or even wrong. By specifying performance requirements, on the other hand, and leaving the means to achieve these to the manufacturer, any solution that satisfies these performance requirements will be acceptable. Treating specification as a means to an end thus represents a genuine ergonomic perspective.
Another standard with a work system approach is under preparation in SC 4, which relates to the design of control rooms, for instance, for process industries or power stations. A multipart standard (ISO 11064) is expected to be prepared as a result, with the different parts dealing with such aspects of control-room design as layout, operator workstation design, and the design of displays and input devices for process control. Because these work items and the approach taken clearly exceed problems of the design of “displays and controls”, SC 4 has been renamed “Human-System Interaction”.
Environmental problems, especially those relating to thermal conditions and communication in noisy environments, are dealt with in SC 5, where standards have been or are being prepared on measurement methods, methods for the estimation of heat stress, conditions of thermal comfort, metabolic heat production, and on auditory and visual danger signals, speech interference level and the assessment of speech communication.
CEN TC 122 covers roughly the same fields of ergonomics standardization, although with a different emphasis and a different structure of its working groups. It is intended, however, that by a division of labour between the ergonomics committees, and mutual acceptance of work results, a general and usable set of ergonomics standards will be developed.
Work systems encompass such macro level organizational variables as the personnel subsystem, the technological subsystem and the external environment. The analysis of work systems is, therefore, essentially an effort to understand the allocation of functions between the worker and the technical outfit and the division of labour between people in a sociotechnical environment. Such an analysis can assist in making informed decisions to enhance systems safety, efficiency in work, technological development and the mental and physical well-being of workers.
Researchers examine work systems according to divergent approaches (mechanistic, biological, perceptual/motor, motivational) with corresponding individual and organizational outcomes (Campion and Thayer 1985). The selection of methods in work systems analysis is dictated by the specific approaches taken and the particular objective in view, the organizational context, the job and human characteristics, and the technological complexity of the system under study (Drury 1987). Checklists and questionnaires are the common means of assembling databases for organizational planners in prioritizing action plans in areas of personnel selection and placement, performance appraisal, safety and health management, worker-machine design and work design or redesign. Inventory methods of checklists, for example the Position Analysis Questionnaire, or PAQ (McCormick 1979), the Job Components Inventory (Banks and Miller 1984), the Job Diagnostic Survey (Hackman and Oldham 1975), and the Multi-method Job Design Questionnaire (Campion 1988) are the more popular instruments, and are directed to a variety of objectives.
The PAQ has six major divisions, comprising 189 behavioural items required for the assessment of job performance and seven supplementary items related to monetary compensation:
The Job Components Inventory Mark II contains seven sections. The introductory section deals with the details of the organization, job descriptions and biographical details of the job holder. Other sections are as follows:
The profile methods have common elements, that is, (1) a comprehensive set of job factors used to select the range of work, (2) a rating scale that permits the evaluation of job demands, and (3) the weighing of job characteristics based on organizational structure and sociotechnical requirements. Les profils des postes, another task profile instrument, developed in the Renault Organization (RNUR 1976), contains a table of entries of variables representing working conditions, and provides respondents with a five-point scale on which they can select the value of a variable that ranges from very satisfactory to very poor by way of registering standardized responses. The variables cover (1) the design of the workstation, (2) the physical environment, (3) the physical load factors, (4) nervous tension, (5) job autonomy, (6) relations, (7) repetitiveness and (8) contents of work.
The AET (Ergonomic Job Analysis) (Rohmert and Landau 1985), was developed based on the stress-strain concept. Each of the 216 items of the AET are coded: one code defines the stressors, indicating whether a work element does or does not qualify as a stressor; other codes define the degree of stress associated with a job; and yet others describe the duration and frequency of stress during the work shift.
The AET consists of three parts:
Broadly speaking, the checklists adopt one of two approaches, (1) the job-oriented approach (e.g., the AET, Les profils des postes) and (2) the worker-oriented approach (e.g., the PAQ). The task inventories and profiles offer subtle comparison of complex tasks and occupational profiling of jobs and determine the aspects of work which are considered a priori as inevitable factors in improving working conditions. The emphasis of the PAQ is on classifying job families or clusters (Fleishman and Quaintence 1984; Mossholder and Arvey 1984; Carter and Biersner 1987), inferring job component validity and job stress (Jeanneret 1980; Shaw and Riskind 1983). From the medical point of view, both the AET and the profile methods allow comparisons of constraints and aptitudes when required (Wagner 1985). The Nordic questionnaire is an illustrative presentation of ergonomic workplace analysis (Ahonen, Launis and Kuorinka 1989), which covers the following aspects:
Among the shortcomings of the general-purpose checklist format employed in ergonomic job analysis are the following:
A systematically constructed checklist obliges us to investigate the factors of work conditions which are visible or easy to modify, and permits us to engage in a social dialogue between employers, job holders and others concerned. One should exercise a degree of caution towards the illusion of simplicity and efficiency of the checklists, and towards their quantifying and technical approaches as well. Versatility in a checklist or questionnaire can be achieved by including specific modules to suit specific objectives. Therefore, the choice of variables is very much linked to the purpose for which the work systems are to be analysed and this determines the general approach for construction of a user-friendly checklist.
The suggested “Ergonomics Checklist” may be adopted for various applications. Data collection and computerized processing of the checklist data are relatively straightforward, by responding to the primary and secondary statements (q.v.).
A broad guideline for a modular-structured work systems checklist is suggested here, covering five major aspects (mechanistic, biological, perceptual/motor, technical and psychosocial). Weighting of the modules varies with the nature of the job(s) to be analysed, the specific features of the country or population under study, organizational priorities and the intended use of the results of the analysis. Respondents mark the “primary statement” as Yes/No. “Yes” answers indicate the apparent absence of a problem, although the advisability of further careful scrutiny should not be ruled out. “No” answers indicate a need for an ergonomics evaluation and improvement. Responses to “secondary statements” are indicated by a single digit on the severity of agreement/disagreement scale illustrated below.
0 Do not know or not applicable
1 Strongly disagree
3 Neither agree nor disagree
5 Strongly agree
A. Organization, worker and the task Your answers/ratings
The checklist designer may provide a sample drawing/photograph of work and
workplace under study.
1. Description of organization and functions.
2. Worker characteristics: A brief account of the work group.
3. Task description: List activities and materials in use. Give some indication of
the work hazards.
B. Mechanistic aspect Your answers/ratings
I. Job Specialization
4.Tasks/work patterns are simple and uncomplicated. Yes/No
If No, rate the following: (Enter 0-5)
4.1 Job assignment is specific to the operative.
4.2 Tools and methods of work are specialized to the purpose of the job.
4.3 Production volume and quality of work.
4.4 Job holder performs multiple tasks.
II. Skill Requirement
5. Job requires simple motor act. Yes/No
If No, rate the following: (Enter 0-5)
5.1 Job requires knowledge and skilful ability.
5.2 Job demands training for skill acquisition.
5.3 Worker makes frequent mistakes at work.
5.4 Job demands frequent rotation, as directed.
5.5 Work operation is machine paced/assisted by automation.
Remarks and suggestions for improvement. Items 4 to 5.5:
q Analyst’s rating Worker’s rating q
C. Biological aspect Your answers/ratings
III. General Physical Activity
6. Physical activity is entirely determined and
regulated by the worker. Yes/No
If No, rate the following: (Enter 0-5)
6.1 Worker maintains target-oriented pace.
6.2 Job implies frequently repeated movements.
6.3 Cardiorespiratory demand of the job:
sedentary/light/moderate/heavy/ extremely heavy.
(What are the heavy work elements?):
6.4 Job demands high muscular strength exertion.
6.5 Job (operation of handle, steering wheel, pedal brake) is predominantly static work.
6.6. Job requires fixed working position (sitting or standing).
IV. Manual Materials Handling (MMH)
Nature of objects handled: animate/inanimate, size and shape.
7. Job requires minimal MMH activity. Yes/No
If No, specify the work:
7.1 Mode of work: (circle one)
(Specify repetition cycle):
7.2 Load weight (kg): (circle one)
5-10, 10-20, 20-30, 30-40, >>40.
7.3 Subject-load horizontal distance (cm): (circle one)
<25, 25-40, 40-55, 55-70, >70.
7.4 Subject-load height: (circle one)
ground, knee, waist, chest, shoulder level.
7.5 Clothing restricts MMH tasks.
8. Task situation is free from risk of bodily injury. Yes/No
If No, rate the following: (Enter 0-5)
8.1 Task can be modified to reduce the load to be handled.
8.2 Materials can be packed in standard sizes.
8.3 Size/position of handles on objects may be improved.
8.4 Workers do not adopt safer methods of load handling.
8.5 Mechanical aids may reduce bodily strains.
List each item if hoists or other handling aids are available.
Suggestions for improvement, Items 6 to 8.5:
V. Workplace/Workspace Design
Workplace may be diagrammatically illustrated, showing human reach and
9. Workplace is compatible with human dimensions. Yes/No
If No, rate the following: (Enter 0-5)
9.1 Work distance is away from normal reach in the horizontal or vertical plane (>60 cm).
9.2 Height of work desk/equipment is fixed or minimally adjustable.
9.3 No space for subsidiary operations (e.g., inspection and maintenance).
9.4 Workstations have obstacles, protruding parts or sharp edges.
9.5 Work surface floors are slippery, uneven, cluttered or unstable.
10. Seating arrangement is adequate (e.g., comfortable chair,
good postural support). Yes/No
If No, the causes are: (Enter 0-5)
10.1 Seat dimensions (e.g., seat height, back rest) mismatch with human dimensions.
10.2 Minimum adjustability of seat.
10.3 Workseat provides no hold/support (e.g., by means of vertical edges/extra stiff covering) to work with the machinery.
10.4 Absence of vibration damping mechanism in the workseat.
11. Sufficient auxiliary support is available for safety
at the workplace. Yes/No
If No, mention the following: (Enter 0-5)
11.1 Non-availability of storage space for tools, personal articles.
11.2 Doorways, entrance/exit routes, or corridors are restricted.
11.3 Design mismatch of handles, ladders, staircases, handrails.
11.4 Handholds and footholds demand awkward position of limbs.
11.5 Supports are unrecognizable by their place, form or construction.
11.6 Limited use of gloves/footwear to work and operate equipment controls.
Suggestions for improvement, Items 9 to 11.6:
VI. Work Posture
12. Job allows a relaxed work posture. Yes/No
If No, rate the following: (Enter 0-5)
12.1 Working with arms above shoulder and/or away from the body.
12.2 Hyperextension of wrist and demand of high strength.
12.3 Neck/shoulder are not maintained at an angle of about 15°.
12.4 Back bent and twisted.
12.5 Hips and legs are not well supported in seated position.
12.6 One-sided and unsymmetrical movement of the body.
12.7 Mention reasons of forced posture:
(1) machine location
(2) seat design,
(3) equipment handling,
12.8 Specify OWAS code. (For a detailed description of the OWAS
method refer to Karhu et al. 1981.)
Suggestions for improvement, Items 12 to 12.7:
VII. Work Environment
(Give measurements where possible)
[Identify noise sources, type and duration of exposure; refer to ILO 1984 code].
13. Noise level is below the maximum Yes/No
sound level recommended. (Use the following table.)
Work requiring no verbal communication
Work requiring verbal communication
Work requiring concentration
under 60 dBA
under 50 dBA
under 45 dBA
over 90 dBA
over 80 dBA
over 75 dBA
Source: Ahonen et al. 1989.
Give your agreement/disagreement score (0-5)
14. Damaging noises are suppressed at the source. Yes/No
If No, rate countermeasures: (Enter 0-5)
14.1 No effective sound isolation present.
14.2 Noise emergency measures are not taken (e.g., restriction of working time, use of personal ear defenders/protectors).
Specify climatic condition.
Radiant Temperature ____
16. Climate is comfortable. Yes/No
If No, rate the following: (Enter 0-5)
16.1 Temperature sensation (circle one):
cool/slightly cool/neutral/warm/very hot
16.2 Ventilation devices (e.g., fans, windows, air conditioners) are not adequate.
16.3 Non-execution of regulatory measures on exposure limits (if available, please elaborate).
16.4 Workers do not wear heat protective/assistive clothing.
16.5 Drinking fountains of cool water are not available nearby.
Workplace/machine(s) are sufficiently illuminated at all times. Yes/No
If No, rate the following: (Enter 0-5)
17.1 Illumination is sufficiently intense.
17.2 Illumination of work area is adequately uniform.
17.3 Flicker phenomena are minimal or absent.
17.4 Shadow formation is nonproblematical.
17.5 Annoying reflected glares are minimal or absent.
17.6 Colour dynamics (visual accentuation, colour warmth) are adequate.
18. DUST, SMOKE, TOXICANTS
Environment is free from excessive dust,
fumes and toxic substances. Yes/No
If No, rate the following: (Enter 0-5)
18.1 Ineffective ventilation and exhaust systems to carry off fumes, smoke and dirt.
18.2 Lack of protection measures against emergency release and contact with dangerous/toxic substances.
List the chemical toxicants:
18.3 Monitoring of the workplace for chemical toxicants is not regular.
18.4 Non-availability of personal protective measures (e.g., gloves, shoes, mask, apron).
Workers are effectively protected against radiation exposure. Yes/No
If No, mention the exposures
(see ISSA checklist, Ergonomics): (Enter 0-5)
19.1 UV radiation (200 nm – 400 nm).
19.2 IR radiation (780 nm – 100 μm).
19.3 Radioactivity/x-ray radiation (<200 nm).
19.4 Microwaves (1 mm – 1 m).
19.5 Lasers (300 nm – 1.4 μm).
19.6 Others (mention):
Machine can be operated without vibration transmission
to the operator’s body. Yes/No
If No, rate the following: (Enter 0-5)
20.1 Vibration is transmitted to the whole body via the feet.
20.2 Vibration transmission occurs through the seat (e.g., mobile machines that are driven with operator seated).
20.3 Vibration is transmitted through the hand-arm system (e.g., power-driven handtools, machines driven when operator is walking).
20.4 Prolonged exposure to continuous/repetitive source of vibration.
20.5 Vibration sources cannot be isolated or eliminated.
20.6 Identify the sources of vibration.
Comments and suggestions, items 13 to 20:
VIII. Work Time Schedule
Indicate work time: work hours/day/week/year, including seasonal work and shift system.
21. Pressure of work time is minimum. Yes/No
If No, rate the following: (Enter 0-5)
21.1 Job requires night work.
21.2 Job involves overtime/extra work time.
Specify average duration:
21.3 Heavy tasks are unevenly distributed throughout the shift.
21.4 People work at a predetermined pace/time limit.
21.5 Fatigue allowances/work-rest patterns are not sufficiently incorporated (use cardio- respiratory criteria on work severity).
Comments and suggestions, items 21 to 21.5:
Analyst’s rating Worker’s ratin
D. Perceptual/motor aspect Your answers/ratings
22. Visual displays (gauges, meters, warning signals)
are easy to read. Yes/No
If No, rate the difficulties: (Enter 0-5)
22.1 Insufficient lighting (refer to item No. 17).
22.2 Awkward head/eye positioning for visual line.
22.3 Display style of numerals/numerical progression creates confusion and causes reading errors.
22.4 Digital displays are not available for accurate reading.
22.5 Large visual distance for reading precision.
22.6 Displayed information is not easily understood.
23. Emergency signals/impulses are easily recognizable. Yes/No
If No, assess the reasons:
23.1 Signals (visual/auditory) do not conform to the work process.
23.2 Flashing signals are out of visual field.
23.3 Auditory display signals are not audible.
24. Groupings of the display features are logical. Yes/No
If No, rate the following:
24.1 Displays are not distinguished by form, position, colour or tone.
24.2 Frequently used and critical displays are removed from the central line of vision.
25. Controls (e.g., switches, knobs, cranes, driving wheels, pedals) are easy to handle. Yes/No
If No, the causes are: (Enter 0-5)
25.1 Hand/foot control positions are awkward.
25.2 Handedness of the controls/tools is incorrect.
25.3 Dimensions of controls do not match the operating body part.
25.4 Controls require high actuating force.
25.5 Controls require high precision and speed.
25.6 Controls are not shape-coded for good grip.
25.7 Controls are not colour/symbol-coded for identification.
25.8 Controls cause unpleasant feeling (warmth, cold, vibration).
26. Displays and controls (combined) are compatible with easy and comfortable human reactions. Yes/No
If No, rate the following: (Enter 0-5)
26.1 Placements are not sufficiently close to each other.
26.2 Display/controls are not sequentially arranged for functions/frequency of use.
26.3 Display/control operations are successive, without enough time span to complete operation (this creates sensory overloading).
26.4 Disharmony in movement direction of display/control (e.g., leftward control movement does not give leftward unit movement).
Comments and suggestions, items 22 to 26.4:
Analyst’s rating Worker’s rating
E. Technical aspect Your answers/ratings
27. Machine (e.g., conveyer trolley, lifting truck, machine tool)
is easy to drive and work with. Yes/No
If No, rate the following: (Enter 0-5)
27.1 Machine is unstable in operation.
27.2 Poor maintenance of the machinery.
27.3 Driving speed of the machine cannot be regulated.
27.4 Steering wheels/handles are operated, from standing position.
27.5 Operating mechanisms hamper body movements in the workspace.
27.6 Risk of injury due to lack of machine guarding.
27.7 Machinery is not equipped with warning signals.
27.8 Machine is poorly equipped for vibration damping.
27.9 Machine noise levels are above legal limits (refer to items No. 13 and 14)
27.10 Poor visibility of machine parts and adjacent area (refer to items No. 17 and 22).
XII. Small Tools/Implements
28. Tools/implements provided to the operatives are
comfortable to work with. Yes/No
If No, rate the following: (Enter 0-5)
28.1 Tool/implement has no carrying strap/back frame.
28.2 Tool cannot be used with alternate hands.
28.3 Heavy weight of the tool causes hyperextension of the wrist.
28.4 Form and position of the handle are not designed for convenient grip.
28.5 Power-driven tool is not designed for two-hand operation.
28.6 Sharp edges/ridges of the tool/equipment can cause injury.
28.7 Harnesses (gloves, etc.) are not regularly used in operating vibrating tool.
28.8 Noise levels of power-driven tool are above acceptable limits
(refer to item No. 13).
Suggestions for improvement, items 27 to 28.8:
XIII. Work Safety
29. Machine safety measures are adequate to prevent
accidents and health hazards. Yes/No
If No, rate the following: (Enter 0-5)
29.1 Machine accessories cannot be fastened and removed easily.
29.2 Dangerous points, moving parts and electrical installations are not adequately guarded.
29.3 Direct/indirect contact of body parts with machinery can cause hazards.
29.4 Difficulty in inspection and maintenance of the machine.
29.5 No clear instructions available for machine operation, maintenance and safety.
Suggestions for improvement, items 29 to 29. 5:
Analyst’s rating Worker’s rating
F. Psychosocial aspect Your answers/ratings
XIV. Job Autonomy
30. Job allows autonomy (e.g., freedom regarding method of work,
performance conditions, time schedule, quality control). Yes/No
If No, the possible causes are: (Enter 0-5)
30.1 No discretion on the starting/finishing times of the job.
30.2 No organizational support as regards calling for assistance at work.
30.3 Insufficient number of people for the task (teamwork).
30.4 Rigidity in work methods and conditions.
XV. Job Feedback (Intrinsic and Extrinsic)
31. Job allows direct feedback of information as to the quality
and quantity of one’s performance. Yes/No
If No, the reasons are: (Enter 0-5)
31.1 No participative role in task information and decision making.
31.2 Constraints of social contact due to physical barriers.
31.3 Communication difficulty due to high noise level.
31.4 Increased attentional demand in machine pacing.
31.5 Other people (managers, co-workers) inform the worker as to his/her effectiveness of job performance.
XVI. Task Variety/Clarity
32. Job has a variety of tasks and calls for spontaneity on the part of the worker. Yes/No
If No, rate the following: (Enter 0-5)
32.1 Job roles and goals are ambiguous.
32.2 Job restrictiveness is imposed by a machine, process or work group.
32.3 Worker-machine relation arouses conflict as to behaviour to be evinced by operator.
32.4 Restricted level of stimulation (e.g., unchanging visual and auditory environment).
32.5 High level of boredom on the job.
32.6 Limited scope for job enlargement.
XVII. Task Identity/Significance
33. Worker is given a batch of tasks Yes/No
and arranges his or her own schedule to complete the work
(e.g., one plans and executes the job and inspects and
manages the products).
Give your agreement/disagreement score (0-5)
34. Job is significant in the organization. Yes/No
It provides acknowledgement and recognition from others.
(Give your agreement/disagreement score)
XVIII. Mental Overload/Underload
35. Job consists of tasks for which clear communication and
unambiguous information support systems are available. Yes/No
If No, rate the following: (Enter 0-5)
35.1 Information supplied in connection with the job is extensive.
35.2 Information handling under pressure is required (e.g., emergency manoeuvering in process control).
35.3 High information-handling workload (e.g., difficult positioning task—no special motivation required).
35.4 Occasional attention is directed to information other than that needed for the actual task.
35.5 Task consists of repetitive simple motor act, with superficial attention needed.
35.6 Tools/equipment are not pre-positioned to avoid mental delay.
35.7 Multiple choices are required in decision making and judging risks.
(Comments and suggestions, items 30 to 35.7)
XIX. Training and Promotion
36. Job has opportunities for associated growth in competence
and task accomplishment. Yes/No
If No, the possible causes are: (Enter 0-5)
36.1 No opportunity for advancement to higher levels.
36.2 No periodic training for operators, specific to jobs.
36.3 Training programs/tools are not easy to learn and use.
36.4 No incentive pay schemes.
XX. Organizational Commitment
37. Defined commitment towards organizational Yes/No
effectiveness, and physical, mental and social well-being.
Assess the degree to which the following are made available: (Enter 0-5)
37.1 Organizational role in individual role conflicts and ambiguities.
37.2 Medical/administrative services for preventive intervention in the case of work hazards.
37.3 Promotional measures to control absenteeism in work group.
37.4 Effective safety regulations.
37.5 Labour inspection and monitoring of better work practices.
37.6 Follow-up action for accident/injury management.
The Summary Evaluation Sheet may be used for profiling and clustering of a selected group of items, which may form the basis for decisions on work systems. The process of analysis is often time-consuming and the users of these instruments must have a sound training in ergonomics both theoretical and practical, in the evaluation of work systems.
SUMMARY EVALUATION SHEET
A. Brief Description of Organization, Worker Characteristics and Task Description
I. Job Specialization
II. Skill Requirement
III. General Physical Activity
IV. Manual Materials Handling
V. workplace/Workplace Design
VI. Work Posture
VII. Work Environment
VIII. Work Time Schedule
XII. Small Tools/Implements
XIII. Work Safety
XIV. Job Autonomy
XV. Job Feedback
XVI. Task Variety/Clarity
XVII. Task Identity/Significance
XVIII. Mental Overload/Underload
XIX. Training and Promotion
XX. Organizational Commitment
Severity Agreement of the Modules
This article is adapted from the 3rd edition of the Encyclopaedia of Occupational Health and Safety.
Anthropometry is a fundamental branch of physical anthropology. It represents the quantitative aspect. A wide system of theories and practice is devoted to defining methods and variables to relate the aims in the different fields of application. In the fields of occupational health, safety and ergonomics anthropometric systems are mainly concerned with body build, composition and constitution, and with the dimensions of the human body’s interrelation to workplace dimensions, machines, the industrial environment and clothing.
An anthropometric variable is a measurable characteristic of the body that can be defined, standardized and referred to a unit of measurement. Linear variables are generally defined by landmarks that can be precisely traced on the body. Landmarks are generally of two types: skeletal-anatomical, which maybe found and traced by feeling bony prominences through the skin, and virtual landmarks that are simply found as maximum or minimum distances using the branches of a caliper.
Anthropometric variables have both genetic and environmental components and may be used to define individual and population variability. The choice of variables must be related to the specific research purpose and standardized with other research in the same field, as the number of variables described in the literature is extremely large, up to 2,200 having been described for the human body.
Anthropometric variables are mainly linear measures, such as heights, distances from landmarks with subject standing or seated in standardized posture; diameters, such as distances between bilateral landmarks; lengths, such as distances between two different landmarks; curved measures, namely arcs, such as distances on the body surface between two landmarks; and girths, such as closed all-around measures on body surfaces, generally positioned at at least one landmark or at a defined height.
Other variables may require special methods and instruments. For instance skinfold thickness is measured by means of special constant pressure calipers. Volumes are measured by calculation or by immersion in water. To obtain full information on body surface characteristics, a computer matrix of surface points may be plotted using biostereometric techniques.
Although sophisticated anthropometric instruments have been described and used with a view to automated data collection, basic anthropometric instruments are quite simple and easy to use. Much care must be taken to avoid common errors resulting from misinterpretation of landmarks and incorrect postures of subjects.
The standard anthropometric instrument is the anthropometer—a rigid rod 2 metres long, with two counter-reading scales, with which vertical body dimensions, such as heights of landmarks from floor or seat, and transverse dimensions, such as diameters, can be taken.
Commonly the rod can be split into 3 or 4 sections which fit into one another. A sliding branch with a straight or curved claw makes it possible to measure distances from the floor for heights, or from a fixed branch for diameters. More elaborate anthropometers have a single scale for heights and diameters to avoid scale errors, or are fitted with digital mechanical or electronic reading devices (figure 1).
Figure 1. An anthropometer
A stadiometer is a fixed anthropometer, generally used only for stature and frequently associated with a weight beam scale.
For transverse diameters a series of calipers may be used: the pelvimeter for measures up to 600 mm and the cephalometer up to 300 mm. The latter is particularly suitable for head measurements when used together with a sliding compass (figure 2).
Figure 2. A cephalometer together with a sliding compass
The foot-board is used for measuring the feet and the head-board provides cartesian co-ordinates of the head when oriented in the “Frankfort plane” (a horizontal plane passing through porion and orbitale landmarks of the head).The hand may be measured with a caliper, or with a special device composed of five sliding rulers.
Skinfold thickness is measured with a constant-pressure skinfold caliper generally with a pressure of 9.81 x 104 Pa (the pressure imposed by a weight of 10 g on an area of 1 mm2).
For arcs and girths a narrow, flexible steel tape with flat section is used. Self-straightening steel tapes must be avoided.
Systems of variables
A system of anthropometric variables is a coherent set of body measurements to solve some specific problem.
In the field of ergonomics and safety the main problem is fitting equipment and work space to humans and tailoring clothes to the right size.
Equipment and work space require mainly linear measures of limbs and body segments that can easily be calculated from landmark heights and diameters, whereas tailoring sizes are based mainly on arcs, girths and flexible tape lengths. Both systems may be combined according to need.
In any case it is absolutely necessary to have a precise space reference for each measurement. The landmarks must therefore be linked by heights and diameters and every arc or girth must have a defined landmark reference. Heights and slopes must be indicated.
In a particular survey the number of variables has to be limited to the minimum so as to avoid undue stress on the subject and operator.
A basic set of variables for work space has been reduced to 33 measured variables (figure 3) plus 20 derived by simple calculation. For a general purpose military survey, Hertzberg and co-workers use 146 variables. For clothes and general biological purposes the Italian Fashion Board (Ente Italiano della Moda) uses a set of 32 general purpose variables and 28 technical ones. The German norm (DIN 61 516) of control body dimensions for clothes includes 12 variables. The recommendation of the International Organization for Standardization (ISO) for anthropometry includes a core list of 36 variables (see table 1). The International Data on Anthropometry tables published by the ILO list 19 body dimensions for the populations of 20 different regions of the world (Jürgens, Aune and Pieper 1990).
Table 1. Basic anthropometric core list
1.1 Forward reach (to hand grip with subject standing upright against a wall)
1.2 Stature (vertical distance from floor to head vertex)
1.3 Eye height (from floor to inner eye corner)
1.4 Shoulder height (from floor to acromion)
1.5 Elbow height (from floor to radial depression of elbow)
1.6 Crotch height (from floor to pubic bone)
1.7 Finger tip height (from floor to grip axis of fist)
1.8 Shoulder breadth (biacromial diameter)
1.9 Hip breadth, standing (the maximum distance across hips)
2.1 Sitting height (from seat to head vertex)
2.2 Eye height, sitting (from seat to inner corner of the eye)
2.3 Shoulder height, sitting (from seat to acromion)
2.4 Elbow height, sitting (from seat to lowest point of bent elbow)
2.5 Knee height (from foot-rest to the upper surface of thigh)
2.6 Lower leg length (height of sitting surface)
2.7 Forearm-hand length (from back of bent elbow to grip axis)
2.8 Body depth, sitting (seat depth)
2.9 Buttock-knee length (from knee-cap to rearmost point of buttock)
2.10 Elbow to elbow breadth (distance between lateral surface of the elbows)
2.11 Hip breadth, sitting (seat breadth)
3.1 Index finger breadth, proximal (at the joint between medial and proximal phalanges)
3.2 Index finger breadth, distal (at the joint between distal and medial phalanges)
3.3 Index finger length
3.4 Hand length (from tip of middle finger to styloid)
3.5 Hand breadth (at metacarpals)
3.6 Wrist circumference
4.1 Foot breadth
4.2 Foot length
5.1 Heat circumference (at glabella)
5.2 Sagittal arc (from glabella to inion)
5.3 Head length (from glabella to opisthocranion)
5.4 Head breadth (maximum above the ear)
5.5 Bitragion arc (over the head between the ears)
6.1 Waist circumference (at the umbilicus)
6.2 Tibial height (from the floor to the highest point on the antero-medial margin of the glenoid of the tibia)
6.3 Cervical height sitting (to the tip of the spinous process of the 7th cervical vertebra).
Source: Adapted from ISO/DP 7250 1980).
Precision and errors
Precision of living body dimensions must be considered in a stochastic manner, because the human body is highly unpredictable, both as a static and as a dynamic structure.
A single individual may grow or change in muscularity and fatness; undergo skeletal changes as a consequence of ageing, disease or accidents; or modify behaviour or posture. Different subjects differ by proportions, not only by general dimensions. Tall stature subjects are not mere enlargements of short ones; constitutional types and somatotypes probably vary more than general dimensions.
The use of mannequins, particularly those representing the standard 5th, 50th and 95th percentiles for fitting trials may be highly misleading, if body variations in body proportions are not taken into consideration.
Errors result from misinterpretation of landmarks and incorrect use of instruments (personal error), imprecise or inexact instruments (instrumental error), or changes in subject posture (subject error—this latter may be due to difficulties of communication if the cultural or linguistic background of the subject differs from that of the operator).
Anthropometric data must be treated by statistical procedures, mainly in the field of inference methods applying univariate (mean, mode, percentiles, histograms, variance analysis, etc.), bivariate (correlation, regression) and multivariate (multiple correlation and regression, factor analysis, etc.) methods. Various graphical methods based on statistical applications have been devised to classify human types (anthropometrograms, morphosomatograms).
Sampling and survey
As anthropometric data cannot be collected for the whole population (except in the rare case of a particularly small population), sampling is generally necessary. A basically random sample should be the starting point of any anthropometric survey. To keep the number of measured subjects to a reasonable level it is generally necessary to have recourse to multiple-stage stratified sampling. This allows the most homogeneous subdivision of the population into a number of classes or strata.
The population may be subdivided by sex, age group, geographical area, social variables, physical activity and so on.
Survey forms have to be designed keeping in mind both measuring procedure and data treatment. An accurate ergonomic study of the measuring procedure should be made in order to reduce the operator’s fatigue and possible errors. For this reason variables must be grouped according to the instrument used and ordered in sequence so as to reduce the number of body flexions the operator has to make.
To reduce the effect of personal error, the survey should be carried out by one operator. If more than one operator has to be used, training is necessary to assure replicability of measurements.
Disregarding the highly criticized concept of “race”, human populations are nevertheless highly variable in size of individuals and in size distribution. Generally human populations are not strictly Mendelian; they are commonly the result of admixture. Sometimes two or more populations, with different origins and adaptation, live together in the same area without interbreeding. This complicates the theoretical distribution of traits. From the anthropometric viewpoint, sexes are different populations. Populations of employees may not correspond exactly to the biological population of the same area as a consequence of possible aptitudinal selection or auto-selection due to job choice.
Populations from different areas may differ as a consequence of different adaptation conditions or biological and genetic structures.
When close fitting is important a survey on a random sample is necessary.
Fitting trials and regulation
The adaptation of work space or equipment to the user may depend not only on the bodily dimensions, but also on such variables as tolerance of discomfort and nature of activities, clothing, tools and environmental conditions. A combination of a check list of relevant factors, a simulator and a series of fitting trials using a sample of subjects chosen to represent the range of body sizes of the expected user population can be used.
The aim is to find tolerance ranges for all subjects. If the ranges overlap it is possible to select a narrower final range which is not outside the tolerance limits of any subject. If there is no overlap it will be necessary to make the structure adjustable or to provide it in different sizes. If more than two dimensions are adjustable a subject may not be able to decide which of the possible adjustments will fit him best.
Adjustability can be a complicated matter, especially when uncomfortable postures result in fatigue. Precise indications must therefore be given to the user who frequently knows little or nothing about his own anthropometric characteristics. In general an accurate design should reduce the need for adjustment to the minimum. In any case it should constantly be kept in mind what is involved is anthropometrics, not merely engineering.
Static anthropometrics may give wide information about movement if an adequate set of variables has been chosen. Nevertheless when movements are complicated and a close fit with the industrial environment is desirable, as in most user-machine and human-vehicle interfaces, an exact survey of postures and movements is necessary. This may be done with suitable mock-ups that allow tracing of reach lines or by photography. In this case a camera fitted with telephoto lens and an anthropometric rod, placed in the sagittal plane of the subject, allows standardized photographs with little distortion of image. Small labels on subjects’ articulations make exact tracing of movements possible.
Another way of studying movements is to formalize postural changes according to a series of horizontal and vertical planes passing through the articulations. Again, using computerized human models with computer-aided design (CAD) systems is a feasible way to include dynamic anthropometrics in ergonomic workplace design.
Muscular Work in Occupational Activities
In industrialized countries around 20% of workers are still employed in jobs requiring muscular effort (Rutenfranz et al. 1990). The number of conventional heavy physical jobs has decreased, but, on the other hand, many jobs have become more static, asymmetrical and stationary. In developing countries, muscular work of all forms is still very common.
Muscular work in occupational activities can be roughly divided into four groups: heavy dynamic muscle work, manual materials handling, static work and repetitive work. Heavy dynamic work tasks are found in forestry, agriculture and the construction industry, for example. Materials handling is common, for example, in nursing, transportation and warehousing, while static loads exist in office work, the electronics industry and in repair and maintenance tasks. Repetitive work tasks can be found in the food and wood-processing industries, for example.
It is important to note that manual materials handling and repetitive work are basically either dynamic or static muscular work, or a combination of these two.
Physiology of Muscular Work
Dynamic muscular work
In dynamic work, active skeletal muscles contract and relax rhythmically. The blood flow to the muscles is increased to match metabolic needs. The increased blood flow is achieved through increased pumping of the heart (cardiac output), decreased blood flow to inactive areas, such as kidneys and liver, and increased number of open blood vessels in the working musculature. Heart rate, blood pressure, and oxygen extraction in the muscles increase linearly in relation to working intensity. Also, pulmonary ventilation is heightened owing to deeper breathing and increased breathing frequency. The purpose of activating the whole cardio-respiratory system is to enhance oxygen delivery to the active muscles. The level of oxygen consumption measured during heavy dynamic muscle work indicates the intensity of the work. The maximum oxygen consumption (VO2max) indicates the person’s maximum capacity for aerobic work. Oxygen consumption values can be translated to energy expenditure (1 litre of oxygen consumption per minute corresponds to approximately 5 kcal/min or 21 kJ/min).
In the case of dynamic work, when the active muscle mass is smaller (as in the arms), maximum working capacity and peak oxygen consumption are smaller than in dynamic work with large muscles. At the same external work output, dynamic work with small muscles elicits higher cardio-respiratory responses (e.g., heart rate, blood pressure) than work with large muscles (figure 1).
Figure 1. Static versus dynamic work
Static muscle work
In static work, muscle contraction does not produce visible movement, as, for example, in a limb. Static work increases the pressure inside the muscle, which together with the mechanical compression occludes blood circulation partially or totally. The delivery of nutrients and oxygen to the muscle and the removal of metabolic end-products from the muscle are hampered. Thus, in static work, muscles become fatigued more easily than in dynamic work.
The most prominent circulatory feature of static work is a rise in blood pressure. Heart rate and cardiac output do not change much. Above a certain intensity of effort, blood pressure increases in direct relation to the intensity and the duration of the effort. Furthermore, at the same relative intensity of effort, static work with large muscle groups produces a greater blood pressure response than does work with smaller muscles. (See figure 2)
Figure 2. The expanded stress-strain model modified from Rohmert (1984)
In principle, the regulation of ventilation and circulation in static work is similar to that in dynamic work, but the metabolic signals from the muscles are stronger, and induce a different response pattern.
Consequences of Muscular Overload in Occupational Activities
The degree of physical strain a worker experiences in muscular work depends on the size of the working muscle mass, the type of muscular contractions (static, dynamic), the intensity of contractions, and individual characteristics.
When muscular workload does not exceed the worker’s physical capacities, the body will adapt to the load and recovery is quick when the work is stopped. If the muscular load is too high, fatigue will ensue, working capacity is reduced, and recovery slows down. Peak loads or prolonged overload may result in organ damage (in the form of occupational or work-related diseases). On the other hand, muscular work of certain intensity, frequency, and duration may also result in training effects, as, on the other hand, excessively low muscular demands may cause detraining effects. These relationships are represented by the so-called expanded stress-strain concept developed by Rohmert (1984) (figure 3).
Figure 3. Analysis of acceptable workloads
In general, there is little epidemiological evidence that muscular overload is a risk factor for diseases. However, poor health, disability and subjective overload at work converge in physically demanding jobs, especially with older workers. Furthermore, many risk factors for work-related musculoskeletal diseases are connected to different aspects of muscular workload, such as the exertion of strength, poor working postures, lifting and sudden peak loads.
One of the aims of ergonomics has been to determine acceptable limits for muscular workloads which could be applied for the prevention of fatigue and disorders. Whereas the prevention of chronic effects is the focus of epidemiology, work physiology deals mostly with short-term effects, that is, fatigue in work tasks or during a work day.
Acceptable Workload in Heavy Dynamic Muscular Work
The assessment of acceptable workload in dynamic work tasks has traditionally been based on measurements of oxygen consumption (or, correspondingly, energy expenditure). Oxygen consumption can be measured with relative ease in the field with portable devices (e.g., Douglas bag, Max Planck respirometer, Oxylog, Cosmed), or it can be estimated from heart rate recordings, which can be made reliably at the workplace, for example, with the SportTester device. The use of heart rate in the estimation of oxygen consumption requires that it be individually calibrated against measured oxygen consumption in a standard work mode in the laboratory, i.e., the investigator must know the oxygen consumption of the individual subject at a given heart rate. Heart rate recordings should be treated with caution because they are also affected by such factors as physical fitness, environmental temperature, psychological factors and size of active muscle mass. Thus, heart rate measurements can lead to overestimates of oxygen consumption in the same way that oxygen consumption values can give rise to underestimates of global physiological strain by reflecting only energy requirements.
Relative aerobic strain (RAS) is defined as the fraction (expressed as a percentage) of a worker’s oxygen consumption measured on the job relative to his or her VO2max measured in the laboratory. If only heart rate measurements are available, a close approximation to RAS can be made by calculating a value for percentage heart rate range (% HR range) with the so-called Karvonen formula as in figure 3.
VO2max is usually measured on a bicycle ergometer or treadmill, for which the mechanical efficiency is high (20-25%). When the active muscle mass is smaller or the static component is higher, VO2max and mechanical efficiency will be smaller than in the case of exercise with large muscle groups. For example, it has been found that in the sorting of postal parcels the VO2max of workers was only 65% of the maximum measured on a bicycle ergometer, and the mechanical efficiency of the task was less than 1%. When guidelines are based on oxygen consumption, the test mode in the maximal test should be as close as possible to the real task. This goal, however, is difficult to achieve.
According to Åstrand’s (1960) classical study, RAS should not exceed 50% during an eight-hour working day. In her experiments, at a 50% workload, body weight decreased, heart rate did not reach steady state and subjective discomfort increased during the day. She recommended a 50% RAS limit for both men and women. Later on she found that construction workers spontaneously chose an average RAS level of 40% (range 25-55%) during a working day. Several more recent studies have indicated that the acceptable RAS is lower than 50%. Most authors recommend 30-35% as an acceptable RAS level for the entire working day.
Originally, the acceptable RAS levels were developed for pure dynamic muscle work, which rarely occurs in real working life. It may happen that acceptable RAS levels are not exceeded, for example, in a lifting task, but the local load on the back may greatly exceed acceptable levels. Despite its limitations, RAS determination has been widely used in the assessment of physical strain in different jobs.
In addition to the measurement or estimation of oxygen consumption, other useful physiological field methods are also available for the quantification of physical stress or strain in heavy dynamic work. Observational techniques can be used in the estimation of energy expenditure (e.g., with the aid of the Edholm scale) (Edholm 1966). Rating of perceived exertion (RPE) indicates the subjective accumulation of fatigue. New ambulatory blood pressure monitoring systems allow more detailed analyses of circulatory responses.
Acceptable Workload in Manual Materials Handling
Manual materials handling includes such work tasks as lifting, carrying, pushing and pulling of various external loads. Most of the research in this area has focused on low back problems in lifting tasks, especially from the biomechanical point of view.
A RAS level of 20-35% has been recommended for lifting tasks, when the task is compared to an individual maximum oxygen consumption obtained from a bicycle ergometer test.
Recommendations for a maximum permissible heart rate are either absolute or related to the resting heart rate. The absolute values for men and women are 90-112 beats per minute in continuous manual materials handling. These values are about the same as the recommended values for the increase in heart rate above resting levels, that is, 30 to 35 beats per minute. These recommendations are also valid for heavy dynamic muscle work for young and healthy men and women. However, as mentioned previously, heart rate data should be treated with caution, because it is also affected by other factors than muscle work.
The guidelines for acceptable workload for manual materials handling based on biomechanical analyses comprise several factors, such as weight of the load, handling frequency, lifting height, distance of the load from the body and physical characteristics of the person.
In one large-scale field study (Louhevaara, Hakola and Ollila 1990) it was found that healthy male workers could handle postal parcels weighing 4 to 5 kilograms during a shift without any signs of objective or subjective fatigue. Most of the handling occurred below shoulder level, the average handling frequency was less than 8 parcels per minute and the total number of parcels was less than 1,500 per shift. The mean heart rate of the workers was 101 beats per minute and their mean oxygen consumption 1.0 l/min, which corresponded to 31% RAS as related to bicycle maximum.
Observations of working postures and use of force carried out for example according to OWAS method (Karhu, Kansi and Kuorinka 1977), ratings of perceived exertion and ambulatory blood pressure recordings are also suitable methods for stress and strain assessments in manual materials handling. Electromyography can be used to assess local strain responses, for example in arm and back muscles.
Acceptable Workload for Static Muscular Work
Static muscular work is required chiefly in maintaining working postures. The endurance time of static contraction is exponentially dependent on the relative force of contraction. This means, for example, that when the static contraction requires 20% of the maximum force, the endurance time is 5 to 7 minutes, and when the relative force is 50%, the endurance time is about 1 minute.
Older studies indicated that no fatigue will be developed when the relative force is below 15% of the maximum force. However, more recent studies have indicated that the acceptable relative force is specific to the muscle or muscle group, and is 2 to 5% of the maximum static strength. These force limits are, however, difficult to use in practical work situations because they require electromyographic recordings.
For the practitioner, fewer field methods are available for the quantification of strain in static work. Some observational methods (e.g., the OWAS method) exist to analyse the proportion of poor working postures, that is, postures deviating from normal middle positions of the main joints. Blood pressure measurements and ratings of perceived exertion may be useful, whereas heart rate is not so applicable.
Acceptable Workload in Repetitive Work
Repetitive work with small muscle groups resembles static muscle work from the point of view of circulatory and metabolic responses. Typically, in repetitive work muscles contract over 30 times per minute. When the relative force of contraction exceeds 10% of the maximum force, endurance time and muscle force start to decrease. However, there is wide individual variation in endurance times. For example, the endurance time varies between two to fifty minutes when the muscle contracts 90 to 110 times per minute at a relative force level of 10 to 20% (Laurig 1974).
It is very difficult to set any definitive criteria for repetitive work, because even very light levels of work (as with the use of a microcomputer mouse) may cause increases in intramuscular pressure, which may sometimes lead to swelling of muscle fibres, pain and reduction in muscle strength.
Repetitive and static muscle work will cause fatigue and reduced work capacity at very low relative force levels. Therefore, ergonomic interventions should aim to minimize the number of repetitive movements and static contractions as far as possible. Very few field methods are available for strain assessment in repetitive work.
Prevention of Muscular Overload
Relatively little epidemiological evidence exists to show that muscular load is harmful to health. However, work physiological and ergonomic studies indicate that muscular overload results in fatigue (i.e., decrease in work capacity) and may reduce productivity and quality of work.
The prevention of muscular overload may be directed to the work content, the work environment and the worker. The load can be adjusted by technical means, which focus on the work environment, tools, and/or the working methods. The fastest way to regulate muscular workload is to increase the flexibility of working time on an individual basis. This means designing work-rest regimens which take into account the workload and the needs and capacities of the individual worker.
Static and repetitive muscular work should be kept at a minimum. Occasional heavy dynamic work phases may be useful for the maintenance of endurance type physical fitness. Probably, the most useful form of physical activity that can be incorporated into a working day is brisk walking or stair climbing.
Prevention of muscular overload, however, is very difficult if a worker’s physical fitness or working skills are poor. Appropriate training will improve working skills and may reduce muscular loads at work. Also, regular physical exercise during work or leisure time will increase the muscular and cardio-respiratory capacities of the worker.
A person’s posture at work—the mutual organization of the trunk, head and extremities—can be analysed and understood from several points of view. Postures aim at advancing the work; thus, they have a finality which influences their nature, their time relation and their cost (physiological or otherwise) to the person in question. There is a close interaction between the body’s physiological capacities and characteristics and the requirement of the work.
Musculoskeletal load is a necessary element in body functions and indispensable in well-being. From the standpoint of the design of the work, the question is to find the optimal balance between the necessary and the excessive.
Postures have interested researchers and practitioners for at least the following reasons:
Figure 1. Too high hand positions or forward bending are amont the most commom ways of creating “static” load
Safety, Health and Working Postures
From a safety and health point of view, all the aspects of posture described above may be important. However, postures as a source of musculoskeletal illnesses such as low back diseases have attracted the most attention. Musculoskeletal problems related to repetitive work are also connected to postures.
Low back pain (LBP) is a generic term for various low back diseases. It has many causes and posture is one possible causal element. Epidemiological studies have shown that physically heavy work is conducive to LBP and that postures are one element in this process. There are several possible mechanisms which explain why certain postures may cause LBP. Forward bending postures increase the load on the spine and ligaments, which are especially vulnerable to loads in a twisted posture. External loads, especially dynamic ones, such as those imposed by jerks and slipping, may increase the loads on the back by a large factor.
From a safety and health standpoint, it is important to identify bad postures and other postural elements as part of the safety and health analysis of work in general.
Recording and Measuring Working Postures
Postures can be recorded and measured objectively by the use of visual observation or more or less sophisticated measuring techniques. They can also be recorded by using self-rating schemes. Most methods consider posture as one of the elements in a larger context, for example, as part of the job content—as do the AET and Renault’s Les profils des postes (Landau and Rohmert 1981; RNUR 1976)—or as a starting point for biomechanical calculations that also take into account other components.
In spite of the advancements in measuring technology, visual observation remains, under field conditions, the only practicable means of systematically recording postures. However, the precision of such measurements remains low. In spite of this, postural observations can be a rich source of information on work in general.
The following short list of measuring methods and techniques presents selected examples:
Video is usually an integral part of the recording and analysis process. The US National Institute for Occupational Safety and Health (NIOSH) has presented guidelines for using video methods in hazard analysis (NIOSH 1990).
Biomechanical and anthropometrical computer programs offer specialized tools for analysing some postural elements in the work activity and in the laboratory (e.g., Chaffin 1969).
Factors Affecting Working Postures
Working postures serve a goal, a finality outside themselves. That is why they are related to external working conditions. Postural analysis that does not take into account the work environment and the task itself is of limited interest to ergonomists.
The dimensional characteristics of the workplace largely define the postures (as in the case of a sitting task), even for dynamic tasks (for example, the handling of material in a confined space). The loads to be handled force the body into a certain posture, as does the weight and nature of the working tool. Some tasks require that body weight be used to support a tool or to apply force on the object of the work, as shown, for example in figure 2.
Figure 2. Ergonomic aspects of standing
Individual differences, age and sex influence postures. In fact, it has been found that a “typical” or “best” posture, for example in manual handling, is largely fiction. For each individual and each working situation, there are a number of alternative “best” postures from the standpoint of different criteria.
Job Aids and Supports for Working Postures
Belts, lumbar supports and orthotics have been recommended for tasks with a risk of low back pain or upper-limb musculoskeletal injuries. It has been assumed that these devices give support to muscles, for example, by controlling intra-abdominal pressure or hand movements. They are also expected to limit the range of movement of the elbow, wrist or fingers. There is no evidence that modifying postural elements with these devices would help to avoid musculoskeletal problems.
Postural supports in the workplace and on machinery, such as handles, supporting pads for kneeling, and seating aids, may be useful in alleviating postural loads and pain.
Safety and Health Regulations concerning Postural Elements
Postures or postural elements have not been subject to regulatory activities per se. However, several documents either contain statements which have a bearing on postures or include the issue of postures as an integral element of a regulation. A complete picture of the existing regulatory material is not available. The following references are presented as examples.
Aims and Principles
Biomechanics is a discipline that approaches the study of the body as though it were solely a mechanical system: all parts of the body are likened to mechanical structures and are studied as such. The following analogies may, for example, be drawn:
The main aim of biomechanics is to study the way the body produces force and generates movement. The discipline relies primarily on anatomy, mathematics and physics; related disciplines are anthropometry (the study of human body measurements), work physiology and kinesiology (the study of the principles of mechanics and anatomy in relation to human movement).
In considering the occupational health of the worker, biomechanics helps to understand why some tasks cause injury and ill health. Some relevant types of adverse health effect are muscle strain, joint problems, back problems and fatigue.
Back strains and sprains and more serious problems involving the intervertebral discs are common examples of workplace injuries that can be avoided. These often occur because of a sudden particular overload, but may also reflect the exertion of excessive forces by the body over many years: problems may occur suddenly or may take time to develop. An example of a problem that develops over time is “seamstress’s finger”. A recent description describes the hands of a woman who, after 28 years of work in a clothing factory, as well as sewing in her spare time, developed hardened thickened skin and an inability to flex her fingers (Poole 1993). (Specifically, she suffered from a flexion deformity of the right index finger, prominent Heberden’s nodes on the index finger and thumb of the right hand, and a prominent callosity on the right middle finger due to constant friction from the scissors.) X-ray films of her hands showed severe degenerative changes in the outermost joints of her right index and middle fingers, with loss of joint space, articular sclerosis (hardening of tissue), osteophytes (bony growths at the joint) and bone cysts.
Inspection at the workplace showed that these problems were due to repeated hyperextension (bending up) of the outermost finger joint. Mechanical overload and restriction in blood flow (visible as a whitening of the finger) would be maximal across these joints. These problems developed in response to repeated muscle exertion in a site other than the muscle.
Biomechanics helps to suggest ways of designing tasks to avoid these types of injuries or of improving poorly designed tasks. Remedies for these particular problems are to redesign the scissors and to alter the sewing tasks to remove the need for the actions performed.
Two important principles of biomechanics are:
Figure 1. Skeletal muscles occur in pairs in order to initiate or reverse a movement
Figure 2. Muscle tension varies with muscle length
Second, if the muscle tries to contract at other than the midrange of the movement of the joint, it will operate at a mechanical disadvantage. Figure 3 illustrates the change in mechanical advantage for the elbow in three different positions.
Figure 3. Optimal positions for joint movement
An important criterion for work design follows from these principles: Work should be arranged so that it occurs with the opposing muscles of each joint in relaxed balance. For most joints, this means that the joint should be at about its midrange of movement.
This rule also means that muscle tension will be at a minimum while a task is performed. One example of the infringement of the rule is the overuse syndrome (RSI, or repetitive strain injury) which affects the muscles of the top of the forearm in keyboard operators who habitually operate with the wrist flexed up. Often this habit is forced on the operator by the design of the keyboard and workstation.
The following are some examples illustrating the application of biomechanics.
The optimum diameter of tool handles
The diameter of a handle affects the force that the muscles of the hand can apply to a tool. Research has shown that the optimum handle diameter depends on the use to which the tool is put. For exerting thrust along the line of the handle, the best diameter is one that allows the fingers and thumb to assume a slightly overlapping grip. This is about 40 mm. To exert torque, a diameter of about 50-65 mm is optimal. (Unfortunately, for both purposes most handles are smaller than these values.)
The use of pliers
As a special case of a handle, the ability to exert force with pliers depends on the handle separation, as shown in figure 4.
Figure 4. Grip strength of pliers jaws exerted by male and female users as a function of handle separation
Electromyography is a technique that can be used to measure muscle tension. In a study of the tension in the erector spinae muscles (of the back) of seated subjects, it was found that leaning back (with the backrest inclined) reduced the tension in these muscles. The effect can be explained because the backrest takes more of the weight of the upper body.
X-ray studies of subjects in a variety of postures showed that the position of relaxed balance of the muscles that open and close the hip joint corresponds to a hip angle of about 135º. This is close to the position (128º) naturally adopted by this joint in weightless conditions (in space). In the seated posture, with an angle of 90º at the hip, the hamstring muscles that run over both the knee and hip joints tend to pull the sacrum (the part of the vertebral column that connects with the pelvis) into a vertical position. The effect is to remove the natural lordosis (curvature) of the lumbar spine; chairs should have appropriate backrests to correct for this effort.
Why are screws inserted clockwise? The practice probably arose in unconscious recognition that the muscles that rotate the right arm clockwise (most people are right-handed) are larger (and therefore more powerful) that the muscles that rotate it anticlockwise.
Note that left-handed people will be at a disadvantage when inserting screws by hand. About 9% of the population are left-handed and will therefore require special tools in some situations: scissors and can openers are two such examples.
A study of people using screwdrivers in an assembly task revealed a more subtle relation between a particular movement and a particular health problem. It was found that the greater the elbow angle (the straighter the arm), the more people had inflammation at the elbow. The reason for this effect is that the muscle that rotates the forearm (the biceps) also pulls the head of the radius (lower arm bone) onto the capitulum (rounded head) of the humerus (upper arm bone). The increased force at the higher elbow angle caused greater frictional force at the elbow, with consequent heating of the joint, leading to the inflammation. At the higher angle, the muscle also had to pull with greater force to effect the screwing action, so a greater force was applied than would have been required with the elbow at about 90º. The solution was to move the task closer to the operators to reduce the elbow angle to about 90º.
The cases above demonstrate that a proper understanding of anatomy is required for the application of biomechanics in the workplace. Designers of tasks may need to consult experts in functional anatomy to anticipate the types of problems discussed. (The Pocket Ergonomist (Brown and Mitchell 1986) based on electromyographical research, suggests many ways of reducing physical discomfort at work.)
Manual Material Handling
The term manual handling includes lifting, lowering, pushing, pulling, carrying, moving, holding and restraining, and encompasses a large part of the activities of working life.
Biomechanics has obvious direct relevance to manual handling work, since muscles must move to carry out tasks. The question is: how much physical work can people be reasonably expected to do? The answer depends on the circumstances; there are really three questions that need to be asked. Each one has an answer that is based on scientifically researched criteria:
There is a need for these three different criteria because there are three broadly different reactions that can occur to lifting tasks: if the work goes on all day, the concern will be how the person feels about the task—the psychophysical criterion; if the force to be applied is large, the concern would be that muscles and joints are not overloaded to the point of damage—the biomechanical criterion; and if the rate of work is too great, then it may well exceed the physiological criterion, or the aerobic capacity of the person.
Many factors determine the extent of the load placed on the body by a manual handling task. All of them suggest opportunities for control.
Posture and Movements
If the task requires a person to twist or reach forward with a load, the risk of injury is greater. The workstation can often be redesigned to prevent these actions. More back injuries occur when the lift begins at ground level compared to mid-thigh level, and this suggests simple control measures. (This applies to high lifting as well.)
The load itself may influence handling because of its weight and its location. Other factors, such as its shape, its stability, its size and its slipperiness may all affect the ease of a handling task.
Organization and environment.
The way work is organized, both physically and over time (temporally), also influences handling. It is better to spread the burden of unloading a truck in a delivery bay over several people for an hour rather than to ask one worker to spend all day on the task. The environment influences handling—poor light, cluttered or uneven floors and poor housekeeping may all cause a person to stumble.
Personal handling skills, the age of the person and the clothing worn also can influence handling requirements. Education for training and lifting are required both to provide necessary information and to allow time for the development of the physical skills of handling. Younger people are more at risk; on the other hand, older people have less strength and less physiological capacity. Tight clothing can increase the muscle force required in a task as people strain against the tight cloth; classic examples are the nurse’s smock uniform and tight overalls when people do work above their heads.
Recommended Weight Limits
The points mentioned above indicate that it is impossible to state a weight that will be “safe” in all circumstances. (Weight limits have tended to vary from country to country in an arbitrary manner. Indian dockers, for example, were once “allowed” to lift 110 kg, while their counterparts in the former People’s Democratic Republic of Germany were “limited” to 32 kg.) Weight limits have also tended to be too great. The 55 kg suggested in many countries is now thought to be far too great on the basis of recent scientific evidence. The National Institute for Occupational Safety and Health (NIOSH) in the United States has adopted 23 kg as a load limit in 1991 (Waters et al. 1993).
Each lifting task needs to be assessed on its own merits. A useful approach to determining a weight limit for a lifting task is the equation developed by NIOSH:
RWL = LC x HM x VM x DM x AM x CM x FM
RWL = recommended weight limit for the task in question
HM = the horizontal distance from the centre of gravity of the load to the midpoint between the ankles (minimum 15 cm, maximum 80 cm)
VM = the vertical distance between the centre of gravity of the load and the floor at the start of the lift (maximum 175 cm)
DM = the vertical travel of the lift (minimum 25 cm, maximum 200 cm)
AM = asymmetry factor–the angle the task deviates from straight out in front of the body
CM = coupling multiplier–the ability to get a good grip on the item to be lifted, which is found in a reference table
FM = frequency multipliers–the frequency of the lifting.
All variables of length in the equation are expressed in units of centimetres. It should be noted that 23 kg is the maximum weight that NIOSH recommends for lifting. This has been reduced from 40 kg after observation of many people doing many lifting tasks has revealed that the average distance from the body of the start of the lift is 25 cm, not the 15 cm assumed in an earlier version of the equation (NIOSH 1981).
By comparing the weight to be lifted in the task and the RWL, a lifting index (LI) can be obtained according to the relationship:
LI=(weight to be handled)/RWL.
Therefore, a particularly valuable use of the NIOSH equation is the placing of lifting tasks in order of severity, using the lifting index to set priorities for action. (The equation has a number of limitations, however, that need to be understood for its most effective application. See Waters et al. 1993).
Estimating Spinal Compression Imposed by the Task
Computer software is available to estimate the spinal compression produced by a manual handling task. The 2D and 3D Static Strength Prediction Programs from the University of Michigan (“Backsoft”) estimate spinal compression. The inputs required to the program are:
The 2D and 3D programs differ in that the 3D software allows computations applying to postures in three dimensions. The program output gives spinal compression data and lists the percentage of the population selected that would be able to do the particular task without exceeding suggested limits for six joints: ankle, knee, hip, first lumbar disc-sacrum, shoulder and elbow. This method also has a number of limitations that need to be fully understood in order to derive maximum value out of the program.
This article is adapted from the 3rd edition of the Encyclopaedia of Occupational Health and Safety.
The two concepts of fatigue and rest are familiar to all from personal experience. The word “fatigue” is used to denote very different conditions, all of which cause a reduction in work capacity and resistance. The very varied use of the concept of fatigue has resulted in an almost chaotic confusion and some clarification of current ideas is necessary. For a long time, physiology has distinguished between muscular fatigue and general fatigue. The former is an acute painful phenomenon localized in the muscles: general fatigue is characterized by a sense of diminishing willingness to work. This article is concerned only with general fatigue, which may also be called “psychic fatigue” or “nervous fatigue” and the rest that it necessitates.
General fatigue may be due to quite different causes, the most important of which are shown in figure 1. The effect is as if, during the course of the day, all the various stresses experienced accumulate within the organism, gradually producing a feeling of increasing fatigue. This feeling prompts the decision to stop work; its effect is that of a physiological prelude to sleep.
Figure 1. Diagrammatic presentation of the cumulative effect of the everyday causes of fatigue
Fatigue is a salutary sensation if one can lie down and rest. However, if one disregards this feeling and forces oneself to continue working, the feeling of fatigue increases until it becomes distressing and finally overwhelming. This daily experience demonstrates clearly the biological significance of fatigue which plays a part in sustaining life, similar to that played by other sensations such as, for example, thirst, hunger, fear, etc.
Rest is represented in figure 1 as the emptying of a barrel. The phenomenon of rest can take place normally if the organism remains undisturbed or if at least one essential part of the body is not subjected to stress. This explains the decisive part played on working days by all workbreaks, from the short pause during work to the nightly sleep. The simile of the barrel illustrates how necessary it is for normal living to reach a certain equilibrium between the total load borne by the organism and the sum of the possibilities for rest.
Neurophysiological interpretation of fatigue
The progress of neurophysiology during the last few decades has greatly contributed to a better understanding of the phenomena triggered off by fatigue in the central nervous system.
The physiologist Hess was the first to observe that electrical stimulation of certain of the diencephalic structures, and more especially of certain of the structures of the medial nucleus of the of the thalamus, gradually produced an inhibiting effect which showed itself in a deterioration in the capacity for reaction and in a tendency to sleep. If stimulation was continued for a certain time, general relaxation was followed by sleepiness and finally by sleep. It was later proved that, starting from these structures, an active inhibition may extend to the cerebral cortex where all conscious phenomena are centred. This is reflected not only in behaviour, but also in the electrical activity of the cerebral cortex. Other experiments have also succeeded in initiating inhibitions from other sub-cortical regions.
The conclusion which can be drawn from all these studies is that there are structures located in the diencephalon and mesencephalon which represent an effective inhibiting system and which trigger off fatigue with all its accompanying phenomena.
Inhibition and activation
Numerous experiments performed on animals and humans have shown that the general disposition of them both to reaction depends not only on this system of inhibition, but essentially also on a system functioning in an antagonistic manner, known as the reticular ascending system of activation. We know from experiments that the reticular formation contains structures which control the degree of wakefulness, and consequently the general dispositions to reaction. Nervous links exist between these structures and the cerebral cortex where the activating influences are exerted on the consciousness. Moreover, the activating system receives stimulations from the sensory organs. Other nervous connections transmit impulses from the cerebral cortex—the area of perception and thought—to the activation system. On the basis of these neurophysiological concepts it can be established that external stimuli as well as influences originating in the areas of consciousness may, in passing through the activating system, stimulate a disposition to reaction.
In addition, many other investigations make it possible to conclude that stimulation of the activating system frequently spreads also from the vegetative centres, and cause the organism to orient towards an expenditure of energy, towards work, struggle, flight, etc. (ergotropic conversion of the internal organs). Conversely, it appears that stimulation of the inhibiting system within the sphere of the vegetative nervous system causes the organism to tend towards rest, reconstitution of its reserves of energy, phenomena of assimilation (trophotropic conversion).
By synthesis of all these neurophysiological findings the following conception of fatigue can be established: the state and feeling of fatigue are conditioned by the functional reaction of the consciousness in the cerebral cortex, which is, in turn, governed by two mutually antagonistic systems—the inhibiting system and the activating system. Thus, the disposition of humans to work depends at each moment on the degree of activation of the two systems: if the inhibiting system is dominant, the organism will be in a state of fatigue; when the activating system is dominant, it will exhibit an increased disposition to work.
This psychophysiological conception of fatigue makes it possible to understand certain of its symptoms which are sometimes difficult to explain. Thus, for example, a feeling of fatigue may disappear suddenly when some unexpected outside event occurs or when emotional tension develops. It is clear in both these cases that the activating system has been stimulated. Conversely, if the surroundings are monotonous or work seems boring, the functioning of the activating system is diminished and the inhibiting system becomes dominant. This explains why fatigue appears in a monotonous situation without the organism being subjected to any workload.
Figure 2 depicts diagrammatically the notion of the mutually antagonistic systems of inhibition and activation.
Figure 2. Diagrammatic presentation of the control of disposition to work by means of inhibiting and activating systems
It is a matter of common experience that pronounced fatigue occurring day after day will gradually produce a state of chronic fatigue. The feeling of fatigue is then intensified and comes on not only in the evening after work but already during the day, sometimes even before the start of work. A feeling of malaise, frequently of an emotive nature, accompanies this state. The following symptoms are often observed in persons suffering from fatigue: heightened psychic emotivity (antisocial behaviour, incompatibility), tendency to depression (unmotivated anxiety), and lack of energy with loss of initiative. These psychic effects are often accompanied by an unspecific malaise and manifest themselves by psychosomatic symptoms: headaches, vertigo, cardiac and respiratory functional disturbances, loss of appetite, digestive disorders, insomnia, etc.
In view of the tendency towards morbid symptoms which accompany chronic fatigue, it may justly be called clinical fatigue. There is a tendency towards increased absenteeism, and particularly to more absences for short periods. This would appear to be caused both by the need for rest and by increased morbidity. The state of chronic fatigue occurs particularly among persons exposed to psychic conflicts or difficulties. It is sometimes very difficult to distinguish the external and internal causes. In fact it is almost impossible to distinguish cause and effect in clinical fatigue: a negative attitude towards work, superiors or workplace may just as well be the cause of clinical fatigue as the result.
Research has shown that the switchboard operators and supervisory personnel employed in telecommunications services exhibited a significant increase in physiological symptoms of fatigue after their work (visual reaction time, flicker fusion frequency, dexterity tests). Medical investigations revealed that in these two groups of workers there was a significant increase in neurotic conditions, irritability, difficulty in sleeping and in the chronic feeling of lassitude, by comparison with a similar group of women employed in the technical branches of the postal, telephone and telegraphic services. The accumulation of symptoms was not always due to a negative attitude on the part of the women affected towards their job or their working conditions.
These is no panacea for fatigue but much can be done to alleviate the problem by attention to general working conditions and the physical environment at the workplace. For example much can be achieved by the correct arrangement of hours of work, provision of adequate rest periods and suitable canteens and rest rooms; adequate paid holidays should also be given to workers. The ergonomic study of the workplace can also help in the reduction of fatigue by ensuring that seats, tables and work benches are of suitable dimensions and that the work flow is correctly organized. In addition, noise control, air-conditioning, heating, ventilation and lighting may all have a beneficial effect on delaying the onset of fatigue in workers.
Monotony and tension may also be alleviated by a controlled use of colour and decoration in the surroundings, intervals of music and sometimes breaks for physical exercises for sedentary workers. Training of workers and in particular of supervisory and management staff also play an important part.
Fatigue and recovery are periodic processes in every living organism. Fatigue can be described as a state which is characterized by a feeling of tiredness combined with a reduction or undesired variation in the performance of the activity (Rohmert 1973).
Not all the functions of the human organism become tired as a result of use. Even when asleep, for example, we breathe and our heart is pumping without pause. Obviously, the basic functions of breathing and heart activity are possible throughout life without fatigue and without pauses for recovery.
On the other hand, we find after fairly prolonged heavy work that there is a reduction in capacity—which we call fatigue. This does not apply to muscular activity alone. The sensory organs or the nerve centres also become tired. It is, however, the aim of every cell to balance out the capacity lost by its activity, a process which we call recovery.
Stress, Strain, Fatigue and Recovery
The concepts of fatigue and recovery at human work is closely related to the ergonomic concepts of stress and strain (Rohmert 1984) (figure 1).
Figure 1. Stress, strain and fatigue
Stress means the sum of all parameters of work in the working system influencing people at work, which are perceived or sensed mainly over the receptor system or which put demands on the effector system. The parameters of stress result from the work task (muscular work, non-muscular work—task-oriented dimensions and factors) and from the physical, chemical and social conditions under which the work has to be done (noise, climate, illumination, vibration, shift work, etc.—situation-oriented dimensions and factors).
The intensity/difficulty, the duration and the composition (i.e., the simultaneous and successive distribution of these specific demands) of the stress factors results in an combined stress, which all the exogenous effects of a working system exert on the working person. This combined stress can be actively coped with or passively put up with, specifically depending on the behaviour of the working person. The active case will involve activities directed towards the efficiency of the working system, while the passive case will induce reactions (voluntary or involuntary), which are mainly concerned with minimizing stress. The relation between the stress and activity is decisively influenced by the individual characteristics and needs of the working person. The main factors of influence are those that determine performance and are related to motivation and concentration, and those related to disposition, which can be referred to as abilities and skills.
The stresses relevant to behaviour, which are manifest in certain activities, cause individually different strains. The strains can be indicated by the reaction of physiological or biochemical indicators (e.g., raising the heart rate) or it can be perceived. Thus, the strains are susceptible to “psycho-physical scaling”, which estimates the strain as experienced by the working person. In a behavioural approach, the existence of strain can also be derived from an activity analysis. The intensity with which indicators of strain (physiological-biochemical, behaviouristic or psycho-physical) react depends on the intensity, duration and combination of stress factors as well as on the individual characteristics, abilities, skills and needs of the working person.
Despite constant stresses the indicators derived from the fields of activity, performance and strain may vary over time (temporal effect). Such temporal variations are to be interpreted as processes of adaptation by the organic systems. The positive effects cause a reduction of strain/improvement of activity or performance (e.g., through training). In the negative case, however, they will result in increased strain/reduced activity or performance (e.g., fatigue, monotony).
The positive effects may come into action if the available abilities and skills are improved in the working process itself, e.g., when the threshold of training stimulation is slightly exceeded. The negative effects are likely to appear if so-called endurance limits (Rohmert 1984) are exceeded in the course of the working process. This fatigue leads to a reduction of physiological and psychological functions, which can be compensated by recovery.
To restore the original performance rest allowances or at least periods with less stress are necessary (Luczak 1993).
When the process of adaptation is carried beyond defined thresholds, the employed organic system may be damaged so as to cause a partial or total deficiency of its functions. An irreversible reduction of functions may appear when stress is far too high (acute damage) or when recovery is impossible for a longer time (chronic damage). A typical example of such damage is noise-induced hearing loss.
Models of Fatigue
Fatigue can be many-sided, depending on the form and combi-nation of strain, and a general definition of it is yet not possible. The biological proceedings of fatigue are in general not measurable in a direct way, so that the definitions are mainly oriented towards the fatigue symptoms. These fatigue symptoms can be divided, for example, into the following three categories.
In the process of fatigue all three of these symptoms may play a role, but they may appear at different points in time.
Physiological reactions in organic systems, particularly those involved in the work, may appear first. Later on the feelings of exertion may be affected. Changes in performance are manifested generally in a decreasing regularity of work or in an increasing quantity of errors, although the mean of the performance may not yet be affected. On the contrary, with appropriate motivation the working person may even try to maintain performance through will-power. The next step may be a clear reduction of performance ending with a breakdown of performance. The physiological symptoms may lead to a breakdown of the organism including changes of the structure of personality and in exhaustion. The process of fatigue is explained in the theory of successive destabilization (Luczak 1983).
The principal trend of fatigue and recovery is shown in figure 2.
Figure 2. Principal trend of fatigue and recovery
Prognosis of Fatigue and Recovery
In the field of ergonomics there is a special interest in predicting fatigue dependent on the intensity, duration and composition of stress factors and to determine the necessary recovery time. Table 1 shows those different activity levels and consideration periods and possible reasons of fatigue and different possibilities of recovery.
Table 1. Fatigue and recovery dependent on activity levels
Level of activity
Phases of work life
One work shift
Free time, rest
Part of a task
Change of stress
In ergonomic analysis of stress and fatigue for determining the necessary recovery time, considering the period of one working day is the most important. The methods of such analyses start with the determination of the different stress factors as a function of time (Laurig 1992) (figure 3).
Figure 3. Stress as a function of time
The stress factors are determined from the specific work content and from the conditions of work. Work content could be the production of force (e.g., when handling loads), the coordination of motor and sensory functions (e.g., when assembling or crane operating), the conversion of information into reaction (e.g., when controlling), the transformations from input to output information (e.g., when programming, translating) and the production of information (e.g., when designing, problem solving). The conditions of work include physical (e.g., noise, vibration, heat), chemical (chemical agents) and social (e.g., colleagues, shift work) aspects.
In the easiest case there will be a single important stress factor while the others can be neglected. In those cases, especially when the stress factors results from muscular work, it is often possible to calculate the necessary rest allowances, because the basic concepts are known.
For example, the sufficient rest allowance in static muscle work depends on the force and duration of muscular contraction as in an exponential function linked by multiplication according to the formula:
R.A. = Rest allowance in percentage of t
t = duration of contraction (working period) in minutes
T = maximal possible duration of contraction in minutes
f = the force needed for the static force and
F = maximal force.
The connection between force, holding time and rest allowances is shown in figure 4.
Figure 4. Percentage rest allowances for various combinations of holding forces and time
Similar laws exist for heavy dynamic muscular work (Rohmert 1962), active light muscular work (Laurig 1974) or different industrial muscular work (Schmidtke 1971). More rarely you find comparable laws for non-physical work, e.g., for computing (Schmidtke 1965). An overview of existing methods for determining rest allowances for mainly isolated muscle and non-muscle work is given by Laurig (1981) and Luczak (1982).
More difficult is the situation where a combination of different stress factors exists, as shown in figure 5, which affect the working person simultaneously (Laurig 1992).
Figure 5. The combination of two stress factors
The combination of two stress factors, for example, can lead to different strain reactions depending on the laws of combination. The combined effect of different stress factors can be indifferent, compensatory or cumulative.
In the case of indifferent combination laws, the different stress factors have an effect on different subsystems of the organism. Each of these subsystems can compensate for the strain without the strain being fed into a common subsystem. The overall strain depends on the highest stress factor, and thus laws of superposition are not needed.
A compensatory effect is given when the combination of different stress factors leads to a lower strain than does each stress factor alone. The combination of muscular work and low temperatures can reduce the overall strain, because low temperatures allow the body to lose heat which is produced by the muscular work.
A cumulative effect arises if several stress factors are superimposed, that is, they must pass through one physiological “bottleneck”. An example is the combination of muscular work and heat stress. Both stress factors affect the circulatory system as a common bottleneck with resultant cumulative strain.
Possible combination effects between muscle work and physical conditions are described in Bruder (1993) (see table 2).
Table 2. Rules of combination effects of two stress factors on strain
Heavy dynamic work
Active light muscle work
Static muscle work
0 indifferent effect; + cumulative effect; – compensatory effect.
Source: Adapted from Bruder 1993.
For the case of the combination of more than two stress factors, which is the normal situation in practice, only limited scientific knowledge is available. The same applies for the successive combination of stress factors, (i.e., the strain effect of different stress factors which affect the worker successively). For such cases, in practice, the necessary recovery time is determined by measuring physiological or psychological parameters and using them as integrating values.
In the 3rd edition of the ILO’s Encyclopaedia, published in 1983, ergonomics was summarized in one article that was only about four pages long. Since the publication of the 3rd edition, there has been a major change in emphasis and in understanding of interrelationships in safety and health: the world is no longer easily classifiable into medicine, safety and hazard prevention. In the last decade almost every branch in the production and service industries has expended great effort in improving productivity and quality. This restructuring process has yielded practical experience which clearly shows that productivity and quality are directly related to the design of working conditions. One direct economical measure of productivity—the costs of absenteeism through illness—is affected by working conditions. Therefore it should be possible to increase productivity and quality and to avoid absenteeism by paying more attention to the design of working conditions.
In sum, the simple hypothesis of modern ergonomics can be stated thus: Pain and exhaustion cause health hazards, wasted productivity and reduced quality, which are measures of the costs and benefits of human work.
This simple hypothesis can be contrasted to occupational medicine which generally restricts itself to establishing the aetiology of occupational diseases. Occupational medicine’s goal is to establish conditions under which the probability of developing such diseases is minimized. Using ergonomic principles these conditions can be most easily formulated in the form of demands and load limitations. Occupational medicine can be summed up as establishing “limitations through medico-scientific studies”. Traditional ergonomics regards its role as one of formulating the methods where, using design and work organization, the limitations established through occupational medicine can be put into practice. Traditional ergonomics could then be described as developing “corrections through scientific studies”, where “corrections” are understood to be all work design recommendations that call for attention to be paid to load limits only in order to prevent health hazards. It is a characteristic of such corrective recommendations that practitioners are finally left alone with the problem of applying them—there is no multidisciplinary team effort.
The original aim of inventing ergonomics in 1857 stands in contrast to this kind of “ergonomics by correction”:
... a scientific approach enabling us to reap, for the benefit of ourselves and others, the best fruits of life’s labour for the minimum effort and maximum satisfaction (Jastrzebowski 1857).
The root of the term “ergonomics” stems from the Greek “nomos” meaning rule, and “ergo” meaning work. One could propose that ergonomics should develop “rules” for a more forward-looking, prospective concept of design. In contrast to “corrective ergonomics”, the idea of prospective ergonomics is based on applying ergonomic recommendations which simultaneously take into consideration profitability margins (Laurig 1992).
The basic rules for the development of this approach can be deduced from practical experience and reinforced by the results of occupational hygiene and ergonomics research. In other words, prospective ergonomics means searching for alternatives in work design which prevent fatigue and exhaustion on the part of the working subject in order to promote human productivity (“... for the benefit of ourselves and others”). This comprehensive approach of prospective ergonomics includes workplace and equipment design as well as the design of working conditions determined by an increasing amount of information processing and a changing work organization. Prospective ergonomics is, therefore, an interdisciplinary approach of researchers and practitioners from a wide range of fields united by the same goal, and one part of a general basis for a modern understanding of occupational safety and health (UNESCO 1992).
Based on this understanding, the Ergonomics chapter in the 4th edition of the ILO Encyclopaedia covers the different clusters of knowledge and experiences oriented toward worker characteristics and capabilities, and aimed at an optimum use of the resource “human work” by making work more “ergonomic”, that is, more humane.
The choice of topics and the structure of articles in this chapter follows the structure of typical questions in the field as practised in industry. Beginning with the goals, principles and methods of ergonomics, the articles which follow cover fundamental principles from basic sciences, such as physiology and psychology. Based on this foundation, the next articles introduce major aspects of an ergonomic design of working conditions ranging from work organization to product design. “Designing for everyone” puts special emphasis on an ergonomic approach that is based on the characteristics and capabilities of the worker, a concept often overlooked in practice. The importance and diversity of ergonomics is shown in two examples at the end of the chapter and can also be found in the fact that many other chapters in this edition of the ILO Encyclopaedia are directly related to ergonomics, such as Heat and Cold, Noise, Vibration, Visual Display Units, and virtually all chapters in the sections Accident and Safety Management and Management and Policy.
Mental Versus Physical Workload
The concept of mental workload (MWL) has become increasingly important since modern semi-automated and computerized technologies may impose severe requirements on human mental or information-processing capabilities within both manufacturing and administrative tasks. Thus, especially for the domains of job analysis, evaluation of job requirements and job design, the conceptualization of mental workload has become even more important than that of traditional physical workload.
Definitions of Mental Workload
There is no agreed-upon definition of mental workload. The main reason is that there are at least two theoretically well-based approaches and definitions: (1) MWL as viewed in terms of the task requirements as an independent, external variable with which the working subjects have to cope more or less efficiently, and (2) MWL as defined in terms of an interaction between task requirements and human capabilities or resources (Hancock and Chignell 1986; Welford 1986; Wieland-Eckelmann 1992).
Although arising from different contexts, both approaches offer necessary and well-founded contributions to different problems.
The requirements resources interaction approach was developed within the context of personality-environment fit/misfit theories which try to explain interindividually differing responses to identical physical and psychosocial conditions and requirements. Thus, this approach may explain individual differences in the patterns of subjective responses to loading requirements and conditions, for example, in terms of fatigue, monotony, affective aversion, burnout or diseases (Gopher and Donchin 1986; Hancock and Meshkati 1988).
The task requirements approach was developed within those parts of occupational psychology and ergonomics which are predominantly engaged in task design, especially in the design of new and untried future tasks, or so-called prospective task design. The background here is the stress-strain concept. Task requirements constitute the stress and the working subjects try to adapt to or to cope with the demands much as they would to other forms of stress (Hancock and Chignell 1986). This task requirements approach tries to answer the question of how to design tasks in advance in order to optimize their later impact on the—often still unknown—employees who will accomplish these future tasks.
There are at least a few common characteristics of both conceptualizations of MWL.
Theoretical Approaches: Requirement-Resources Approaches
From the person-environment fit point of view, MWL and its consequences may be roughly categorized—as is shown in figure 1—into underload, properly fitting load, and overload. This categorization results from the relationships between task requirements and mental capabilities or resources. Task requirements may exceed, fit with or fail to be satisfied by the resources. Both types of misfit may result from quantitative or qualitative modes of misfit and will have qualitatively differing, but in any case negative, consequences (see figure 1).
Figure 1. Types and consequences of requirements-resources relationships
Some theories attempt to define MWL starting from the resource or capacity side of the requirements, namely, resources relationships. These resource theories might be subdivided into resource volume and resource allocation theories (Wieland-Eckelmann 1992). The amount of available capacity may come from a single source (single resource theories) which determines processing. The availability of this resource varies with arousal (Kahneman 1973). Modern multiple resource theories suppose a set of relatively independent processing resources. Thus, performance will depend on the condition whether the same resource or different resources are required simultaneously and concurrently. Different resources are, for example, encoding, processing or responding resources (Gopher and Donchin 1986; Welford 1986). The most critical problem for these types of theories is the reliable identification of one or more well-defined capacities for qualitatively different processing operations.
Resource allocation theories suppose qualitatively changing processing as a function of varying strategies. Depending on the strategies, differing mental processes and representations may be applied for task accomplishment. Thus, not the volume of stable resources but flexible allocation strategies become the key point of interest. Again, however, essential questions—especially concerning the methods of diagnosis of the strategies—remain to be answered.
Assessment of MWL: using requirement-resource approaches
A strict measurement of MWL at present would be impossible since well-defined units of measurement are lacking. But, to be sure, the conceptualization and the instruments for an assessment should meet the general quality criteria of diagnostic approaches, which have objectivity, reliability, validity and usefulness. However, as of now, only a little is known about the overall quality of proposed techniques or instruments.
There are a sizeable number of reasons for the remaining difficulties with assessing MWL according to the requirement-resource approaches (O’Donnell and Eggemeier 1986). An attempt at MWL assessment has to cope with questions like the following: is the task self-intended, following self-set goals, or is it directed with reference to an externally defined order? Which type of capacities (conscious intellectual processing, application of tacit knowledge, etc.) are required, and are they called upon simultaneously or sequentially? Are there different strategies available and, if so, which ones? Which coping mechanisms of a working person might be required?
The most often discussed approaches try to assess MWL in terms of:
Both approaches are heavily dependent on the assumptions of single resource theories and consequently have to struggle with the above-mentioned questions.
Effort assessment. Such effort assessment techniques as, for example, the scaling procedure applied to a perceived correlate of the general central activation, developed and validated by Bartenwerfer (1970), offer verbal scales which may be completed by graphic ones and which grade the unidimensionally varying part of the perceived required effort during task accomplishment. The subjects are requested to describe their perceived effort by means of one of the steps of the scale provided.
The quality criteria mentioned above are met by this technique. Its limitations include the unidimensionality of the scale, covering an essential but questionable part of perceived effort; the limited or absent possibility of forecasting perceived personal task outcomes, for example, in terms of fatigue, boredom or anxiety; and especially the highly abstract or formal character of effort which will identify and explain nearly nothing of the content-dependent aspects of MWL as, for example, any possible useful applications of the qualification or the learning options.
Mental capacity assessment. The mental capacity assessment consists of the dual task techniques and a related data interpretation procedure, called the performance operating characteristic (POC). Dual task techniques cover several procedures. Their common feature is that subjects are requested to perform two tasks simultaneously. The crucial hypothesis is: the less an additional or secondary task in the dual task situation will deteriorate in comparison with the base-line single task situation, the lower the mental capacity requirements of the primary task, and vice versa. The approach is now broadened and various versions of task interference under dual task conditions are investigated. For example, the subjects are instructed to perform two tasks concurrently with graded variations of the priorities of the tasks. The POC curve graphically illustrates the effects of possible dual-task combinations arising from sharing limited resources among the concurrently performed tasks.
The critical assumptions of the approach mainly consist in the suggestions that every task will require a certain share of a stable, limited conscious (versus unconscious, automated, implicit or tacit) processing capacity, in the hypothetical additive relationship of the two capacity requirements, and in the restriction of the approach to performance data only. The latter might be misleading for several reasons. First of all there are substantial differences in the sensitivity of performance data and subjectively perceived data. Perceived load seems to be determined mainly by the amount of required resources, often operationalized in terms of working memory, whereas performance measures seem to be determined predominantly by the efficiency of the sharing of resources, depending on allocation strategies (this is dissociation theory; see Wickens and Yeh 1983). Moreover, individual differences in information processing abilities and personality traits strongly influence the indicators of MWL within the subjective (perceived), performance and psychophysiological areas.
Theoretical Approaches: Task Requirement Approaches
As has been shown, task requirements are multidimensional and, thus, may not be described sufficiently by means of only one dimension, whether it be the perceived effort or the residual conscious mental capacity. A more profound description might be a profile-like one, applying a theoretically selected pattern of graded dimensions of task characteristics. The central issue is thus the conceptualization of “task”, especially in terms of task content, and of “task accomplishment”, especially in terms of the structure and phases of goal-oriented actions. The role of the task is stressed by the fact that even the impact of contextual conditions (like temperature, noise or working hours) on the persons are task-dependent, since they are mediated by the task acting as a gate device (Fisher 1986). Various theoretical approaches sufficiently agree regarding those critical task dimensions, which offer a valid prediction of the task outcome. In any case, task outcome is twofold, since (1) the intended result must be achieved, meeting the performance-outcome criteria, and (2) a number of unintended personal short-term and cumulative long-term side effects will emerge, for example fatigue, boredom (monotony), occupational diseases or improved intrinsic motivation, knowledge or skills.
Assessment of MWL. With task requirement approaches, action-oriented approaches like those of complete versus partialized actions or the motivation potential score (for an elaboration of both see Hacker 1986), propose as indispensable task characteristics for analysis and evaluation at least the following:
The identification of these task characteristics requires the joint procedures of job/task analysis, including document analyses, observations, interviews and group discussions, which must be integrated in a quasi-experimental design (Rudolph, Schönfelder and Hacker 1987). Task analysis instruments which may guide and assist the analysis are available. Some of them assist only the analysis (for example, NASA-TLX Task Load Index, Hart and Staveland, 1988) while others are useful for evaluation and design or redesign. An example here is the TBS-GA (Tätigkeitsbewertungs System für geistige Arbeit [Task Diagnosis Survey—Mental Work]); see Rudolph, Schönfelder and Hacker (1987).
The WHO (World Health Organization) introduced in 1980 a classification of functional limitation in people; the ICIDH (International Classification Impairment, Disability and Handicap). In this classification a difference is made between illness, limitations and handicap.
This reference model was created to facilitate international communication. The model was presented on the one hand to offer a reference framework for policy makers and on the other hand, to offer a reference framework for doctors diagnosing people suffering from the consequences of illness.
Why this reference framework? It arose with the aim of trying to improve and increase the participation of people with long-term limited abilities. Two aims are mentioned:
As of January 1st, 1994 the classification is official. The activities that have followed, are widespread and especially concerned with issues such as: information and educational measures for specific groups; regulations for the protection of workers; or, for instance, demands that companies should employ, for example, at least 5 per cent of workers with a disability. The classification itself leads in the long term to integration and non-discrimination.
Illness strikes each of us. Certain illnesses can be prevented, others not. Certain illnesses can be cured, others not. Where possible illness should be prevented and if possible cured.
Impairment means every absence or abnormality of a psychological, physiological or anatomic structure or function.
Being born with three fingers instead of five does not have to lead to disability. The capabilities of the individual, and the degree of manipulation possible with the three fingers, will determine whether or not the person is disabled. When, however, a fair amount of signal processing is not possible on a central level in the brain, then impairment will certainly lead to disability as at present there is no method to “cure” (solve) this problem for the patient.
Disability describes the functional level of an individual having difficulty in task performance e.g., difficulty standing up from their chair. These difficulties are of course related to the impairment, but also to the circumstances surrounding it. A person who uses a wheelchair and lives in a flat country like the Netherlands has more possibilities for self-transportation than the same person living in a mountainous area like Tibet.
When the problems are placed on a handicap level, it can be determined in which field the main problems are effective e.g., immobility or physical dependency. These can affect work performance; for example the person may not be able to get themselves to work; or, once at work, might need assistance in personal hygiene, etc.
A handicap shows the negative consequences of disability and can only be solved by taking the negative consequences away.
Summary and conclusions
The above-mentioned classification and the policies thereof offer a well defined international workable framework. Any discussion on designing for specific groups will need such a framework in order to define our activities and try to implement these thoughts in design.
The concept of vigilance refers to a human observer’s state of alertness in tasks that demand efficient registration and processing of signals. The main characteristics of vigilance tasks are relatively long durations and the requirement to detect infrequent and unpredictable target stimuli (signals) against a background of other stimulus events.
The prototypical task for vigilance research was that of radar operators. Historically, their apparently unsatisfactory performance during the Second World War has been a major impetus for the extensive study of vigilance. Another major task requiring vigilance is industrial inspection. More generally, all kinds of monitoring tasks which require the detection of relatively infrequent signals embody the risk of failures to detect and to respond to these critical events.
Vigilance tasks make up a heterogeneous set and vary on several dimensions, in spite of their common characteristics. An obviously important dimension is the overall stimulus rate as well as the rate of target stimuli. It is not always possible to define stimulus rate unambiguously. This is the case in tasks which require the detection of target events against continuously presented background stimuli, as in detecting critical values on a set of dials in a monitoring task. A less obviously important distinction is that between successive-discrimination tasks and simultaneous-discrimination tasks. In simultaneous-discrimination tasks both target stimuli and background stimuli are present at the same time, while in successive-discrimination tasks one is presented after the other so that some demands on memory are made. Although most vigilance tasks require detection of visual stimuli, stimuli in other modalities have also been studied. Stimuli can be confined to a single spatial location, or there can be different sources for target stimuli. Target stimuli can differ from background stimuli by physical characteristics, but also by more conceptual ones (like a certain pattern of meter readings that can differ from other patterns). Of course, the conspicuousness of targets can vary: some can be detected easily, while others may be hard to discriminate from background stimuli. Target stimuli can be unique or there can be sets of target stimuli without well-defined boundaries to set them off from background stimuli, as is the case in many industrial inspection tasks. This list of dimensions on which vigilance tasks differ can be expanded, but even this length of the list suffices to emphasize the heterogeneity of vigilance tasks and thus the risks involved in generalizing certain observations across the full set.
Performance Variations and the Vigilance Decrement
The most frequently used performance measure in vigilance tasks is the proportion of target stimuli, for example, faulty products in industrial inspection, that have been detected; this is an estimate of the probability of so-called hits. Those target stimuli that remain unnoticed are called misses. Although the hit rate is a convenient measure, it is somewhat incomplete. There is a trivial strategy that allows one to achieve 100% hits: one only has to classify all stimuli as targets. However, the hit rate of 100% is then accompanied by a false-alarm rate of 100%, that is, not only the target stimuli are correctly detected, but the background stimuli are incorrectly “detected” as well. This line of reasoning makes it quite clear that whenever there are false alarms at all, it is important to know their proportion in addition to the hit rate. Another measure for performance in a vigilance task is the time needed to respond to target stimuli (response time).
Performance in vigilance tasks exhibits two typical attributes. The first one is the low overall level of vigilance performance. It is low in comparison with an ideal situation for the same stimuli (short observation periods, high readiness of the observer for each discrimination, etc.). The second attribute is the so-called vigilance decrement, the decline of performance in the course of the watch which can start within the first few minutes. Both these observations refer to the proportion of hits, but they have also been reported for response times. Although the vigilance decrement is typical of vigilance tasks, it is not universal.
In investigating the causes of poor overall performance and vigilance decrements, a distinction will be made among concepts that are related to the basic characteristics of the task and concepts that are related to organismic and task-unrelated situational factors. Among the task-related factors strategic and non-strategic ones can be distinguished.
Strategic processes in vigilance tasks
The detection of a signal like a faulty product is partly a matter of the observer’s strategy and partly a matter of the signal’s discriminability. This distinction is based on the theory of signal detection (TSD), and some basics of the theory need to be presented in order to highlight the distinction’s importance. Consider a hypothetical variable, defined as “evidence for the presence of a signal”. Whenever a signal is presented, this variable takes on some value, and whenever a background stimulus is presented, it takes on a value that is lower on the average. The value of the evidence variable is assumed to vary across repeated presentations of the signal. Thus it can be characterized by a so-called probability density function as is illustrated in figure 1. Another density function characterizes the values of the evidence variable upon presentation of a background stimulus. When the signals are similar to the background stimuli, the functions will overlap, so that a certain value of the evidence variable can originate either from a signal or from a background stimulus. The particular shape of the density functions of figure 1 is not essential for the argument.
Figure 1. Thresholds and discriminability
The detection response of the observer is based on the evidence variable. It is assumed that a threshold is set so that a detection response is given whenever the value of the evidence variable is above the threshold. As is illustrated in figure 1, the areas under the density functions to the right of the threshold correspond to the probabilities of hits and false alarms. In practice, estimates of the separation of the two functions and the location of the threshold can be derived. The separation of the two density functions characterizes the discriminability of the target stimuli from the background stimuli, while the location of the threshold characterizes the observer’s strategy. Variation of the threshold produces a joint variation of the proportions of hits and false alarms. With a high threshold, the proportions of hits and false alarms will be small, while with a low threshold the proportions will be large. Thus, selection of a strategy (placement of the threshold) essentially is the selection of a certain combination of hit rate and false-alarm rate among the combinations that are possible for a certain discriminability.
Two major factors that influence the location of the threshold are payoffs and signal frequency. The threshold will be set to lower values when there is much to gain from a hit and little to lose from a false alarm, and it will be set to higher values when false alarms are costly and the benefits from hits are small. A low threshold setting can also be induced by a high proportion of signals, while a low proportion of signals tends to induce higher threshold settings. The effect of signal frequency on threshold settings is a major factor for the low overall performance in terms of the proportion of hits in vigilance tasks and for the vigilance decrement.
An account of the vigilance decrement in terms of strategic changes (threshold changes) requires that the reduction of the proportion of hits in the course of the watch is accompanied by a reduction of the proportion of false alarms. This is, in fact, the case in many studies, and it is likely that the overall poor performance in vigilance tasks (in comparison with the optimal situation) does also result, at least partly, from a threshold adjustment. In the course of a watch the relative frequency of detection responses comes to match the relative frequency of targets, and this adjustment implies a high threshold with a relatively small proportion of hits and a relatively small proportion of false alarms as well. Nevertheless there are vigilance decrements that result from changes in discriminability rather than from changes in threshold settings. These have been observed mainly in successive-discrimination tasks with a relatively high rate of stimulus events.
Nonstrategic processes in vigilance tasks
Although part of the overall poor performance in vigilance tasks and many instances of the vigilance decrement can be accounted for in terms of strategic adjustments of the detection threshold to low signal rates, such an account is not complete. There are changes in the observer during a watch that can reduce the discriminability of stimuli or result in apparent threshold shifts that cannot be considered as adaptation to the task characteristics. In the more than 40 years of vigilance research a number of nonstrategic factors that contribute to poor overall performance and to the vigilance decrement have been identified.
A correct response to a target in a vigilance task requires a sufficiently precise sensory registration, an appropriate threshold location, and a link between the perceptual processes and the associated response-related processes. During the watch the observers have to maintain a certain task set, a certain readiness to respond to target stimuli in a certain way. This is a nontrivial requirement, because without a particular task set no observer would respond to target stimuli in the way required. Two major sources of failures are thus inaccurate sensory registration and lapses in the readiness to respond to target stimuli. Major hypotheses to account for such failures will be briefly reviewed.
Detection and identification of a stimulus are faster when there is no temporal or spatial uncertainty about its appearance. Temporal and/or spatial uncertainty is likely to reduce vigilance performance. This is the essential prediction of expectancy theory. Optimal preparedness of the observer requires temporal and spatial certainty; obviously vigilance tasks are less than optimal in this respect. Although the major focus of expectancy theory is on the overall low performance, it can also serve to account for parts of the vigilance decrement. With infrequent signals at random intervals, high levels of preparedness might initially exist at times when no signal is presented; in addition, signals will be presented at low levels of preparedness. This discourages occasional high levels of preparedness in general so that whatever benefits accrue from them will vanish in the course of a watch.
Expectancy theory has a close relation to attentional theories. Variants of attentional theories of vigilance, of course, are related to dominant theories of attention in general. Consider a view of attention as “selection for processing” or “selection for action”. According to this view, stimuli are selected from the environment and processed with high efficiency whenever they serve the currently dominant action plan or task set. As already said, selection will benefit from precise expectations about when and where such stimuli will occur. But stimuli will only be selected if the action plan—the task set—is active. (Drivers of cars, for example, respond to traffic lights, other traffic, etc.; passengers don’t do so normally, although both are in almost the same situation. The critical difference is that between the task sets of the two: only the driver’s task set requires responses to traffic lights.)
Selection of stimuli for processing will suffer when the action plan is temporarily deactivated, that is, when the task set is temporarily absent. Vigilance tasks embody a number of features that discourage continuous maintenance of the task set, like short cycle times for processing stimuli, lack of feedback and little motivational challenge by apparent task difficulty. So-called blockings can be observed in almost all simple cognitive tasks with short cycle times like simple mental arithmetic or rapid serial responses to simple signals. Similar blockings occur in the maintenance of the task set in a vigilance task as well. They are not immediately recognizable as delayed responses because responses are infrequent and targets that are presented during a period of absent task set may no longer be there when the absence is over, so that a miss will be observed instead of a delayed response. Blockings become more frequent with time spent on task. This can give rise to the vigilance decrement. There may be additional reasons for temporary lapses in the availability of the appropriate task set, for example, distraction.
Certain stimuli are not selected in the service of the current action plan, but by virtue of their own characteristics. These are stimuli that are intense, novel, moving toward the observer, have an abrupt onset or for any other reason might require immediate action no matter what the current action plan of the observer is. There is little risk of not detecting such stimuli. They attract attention automatically, as is indicated, for example, by the orienting response, which includes a shift of the direction of the gaze toward the stimulus source. However, answering an alarm bell is not normally considered a vigilance task. In addition to stimuli that attract attention by their own characteristics, there are stimuli that are processed automatically as a consequence of practice. They seem to “pop out” from the environment. This kind of automatic processing requires extended practice with a so-called consistent mapping, that is, a consistent assignment of responses to stimuli. The vigilance decrement is likely to be small or even absent once automatic processing of stimuli has been developed.
Finally, vigilance performance suffers from a lack of arousal. This concept refers in a rather global manner to the intensity of neural activity, ranging from sleep through normal wakefulness to high excitement. One of the factors that is thought to affect arousal is external stimulation, and this is fairly low and uniform in most vigilance tasks. Thus, the intensity of central nervous system activity can decline overall over the course of a watch. An important aspect of arousal theory is that it links vigilance performance to various task-unrelated situational factors and factors related to the organism.
The Influence of Situational and Organismic Factors
Low arousal contributes to poor performance in vigilance tasks. Thus performance can be enhanced by situational factors that tend to enhance arousal, and it can be reduced by all measures that reduce the level of arousal. On balance, this generalization is mostly correct for the overall performance level in vigilance tasks, but the effects on the vigilance decrement are absent or less reliably observed across different kinds of manipulation of arousal.
One way to raise the level of arousal is the introduction of additional noise. However, the vigilance decrement is generally unaffected, and with respect to overall performance the results are inconsistent: enhanced, unchanged and reduced performance levels have all been observed. Perhaps the complex nature of noise is relevant. For example, it can be affectively neutral or annoying; it cannot only be arousing, but also be distracting. More consistent are the effects of sleep deprivation, which is “de-arousing”. It generally reduces vigilance performance and has sometimes been seen to enhance the vigilance decrement. Appropriate changes of vigilance performance have also been observed with depressant drugs like benzodiazepines or alcohol and stimulant drugs like amphetamine, caffeine or nicotine.
Individual differences are a conspicuous feature of performance in vigilance tasks. Although individual differences are not consistent across all sorts of vigilance tasks, they are fairly consistent across similar ones. There is only little or no effect of sex and general intelligence. With respect to age, vigilance performance increases during childhood and tends to decline beyond the age of sixty. In addition there is a good chance that introverts will show better performance than extroverts.
The Enhancement of Vigilance Performance
The existing theories and data suggest some means to enhance vigilance performance. Depending on the amount of specificity of the suggestions, it is not difficult to compile lists of various lengths. Some rather broad suggestions are given below that have to be fitted to specific task requirements. They are related to the ease of perceptual discriminations, the appropriate strategic adjustments, the reduction of uncertainty, the avoidance of the effects of attentional lapses and the maintenance of arousal.
Vigilance tasks require discriminations under non-optimal conditions. Thus one is well advised in making the discriminations as easy as possible, or the signals as conspicuous as possible. Measures related to this general goal can be straightforward (like appropriate lighting or longer inspection times per product) or more sophisticated, including special devices to enhance the conspicuousness of targets. Simultaneous comparisons are easier than successive ones, so the availability of a reference standard can be helpful. By means of technical devices it is sometimes possible to present the standard and the object to be examined in rapid alternation, so that differences will appear as motions in the display or other changes for which the visual system is particularly sensitive.
To counteract the strategic changes of the threshold that lead to a relatively low proportion of correct detections of targets (and for making the task less boring in terms of the frequency of actions to be taken) the suggestion has been made to introduce fake targets. However, this seems not to be a good recommendation. Fake targets will increase the proportion of hits overall, but at the cost of more frequent false alarms. In addition, the proportion of undetected targets among all stimuli that are not responded to (the outgoing faulty material in an industrial inspection task) will not necessarily be reduced. Better suited seems to be explicit knowledge about the relative importance of hits and false alarms and perhaps other measures to obtain an appropriate placement of the threshold for deciding between “good” and “bad”.
Temporal and spatial uncertainty are important determinants of poor vigilance performance. For some tasks spatial uncertainty can be reduced by way of defining a certain position of the object to be inspected. However, little can be done about temporal uncertainty: the observer would be unnecessary in a vigilance task if the occurrence of a target could be signalled in advance of its presentation. One thing that can be done in principle, however, is to mix objects to be inspected if faults tend to occur in bunches; this serves to avoid very long intervals without targets as well as very short intervals.
There are some obvious suggestions for the reduction of attentional lapses or at least their impact on performance. By proper training, some kind of automatic processing of targets can perhaps be obtained provided that the background and target stimuli are not too variable. The requirement for a sustained maintenance of the task set can be avoided by means of frequent short breaks, job rotation, job enlargement or job enrichment. Introduction of variety can be as simple as having the inspector himself or herself getting the material to be inspected from a box or other location. This also introduces self-pacing, which may help in avoiding signal presentations during temporary deactivations of the task set. Sustained maintenance of task set can be supported by means of feedback, indicated interest by supervisors and operator’s awareness of the importance of the task. Of course, accurate feedback of performance level is not possible in typical vigilance tasks; however, even inaccurate or incomplete feedback can be helpful as far as the observer’s motivation is concerned.
There are some measures that can be taken to maintain a sufficient level of arousal. Continuous use of drugs may exist in practice, but is never found among recommendations. Some background music can be useful, but can also have an opposite effect. Social isolation during vigilance tasks should mostly be avoided, and during times of day with low levels of arousal like the late hours of the night, supportive measures such as short watches are particularly important.