" DISCLAIMER: The ILO does not take responsibility for content presented on this web portal that is presented in any language other than English, which is the language used for the initial production and peer-review of original content. Certain statistics have not been updated since the production of the 4th edition of the Encyclopaedia (1998)."
Physical and Physiological Aspects
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 to the body. Landmarks are generally of two types: skeletal-anatomical, which may be 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).
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).
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 problems.
In the field of ergonomics and safety, the main problem is fitting equipment and workspace to humans and tailoring clothes to the right size.
Equipment and workspace 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 workspace has been reduced to 33 measured variables (figure 3) plus 20 derived by a 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).
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 Handbreadth (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
The 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 aging, disease or accidents; or modify behavior 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 the 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 workspace 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 checklist 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 that 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 a telephoto lens and an anthropometric rod, placed in the sagittal plane of the subject, allows standardized photographs with little distortion of the image. Small labels on subjects’ articulations make the 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).
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)
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).
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:
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.
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:
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.
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.
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, 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 muscle 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.
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 work breaks, 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 thalamus, gradually produced an inhibiting effect which showed itself in a deterioration in the capacity for reaction and in a tendency to sleep. If the 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 centered. 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 subcortical 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 that control the degree of wakefulness, and consequently the general dispositions to a 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 stimulation 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 a reaction.
In addition, many other investigations make it possible to conclude that stimulation of the activating system frequently spreads also from the vegetative centers, and cause the organism to orient towards the 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.
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 that 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 their job or their working conditions.
There 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 restrooms; 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 workbenches are of suitable dimensions and that the workflow 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 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).
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 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.
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).
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.
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).
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.