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:

1. A posture is the source of musculoskeletal load. Except for relaxed standing, sitting and lying horizontally, muscles have to create forces to balance the posture and/or control movements. In classical heavy tasks, for example in the construction industry or in the manual handling of heavy materials, external forces, both dynamic and static, add to the internal forces in the body, sometimes creating high loads which may exceed the capacity of the tissues. (See figure 1) Even in relaxed postures, when muscle work approaches zero, tendons and joints may be loaded and show signs of fatigue. A job with low apparent loading—an example being that of a microscopist—may become tedious and strenuous when it is carried out over a long period of time.
2. Posture is closely related to balance and stability. In fact, posture is controlled by several neural reflexes where input from tactile sensations and visual cues from the surroundings play an important role. Some postures, like reaching objects from a distance, are inherently unstable. Loss of balance is a common immediate cause of work accidents. Some work tasks are performed in an environment where stability cannot always be guaranteed, for example, in the construction industry.
3. Posture is the basis of skilled movements and visual observation. Many tasks require fine, skilled hand movements and close observation of the object of the work. In such cases, posture becomes the platform of these actions. Attention is directed to the task, and the postural elements are enlisted to support the tasks: the posture becomes motionless, the mus-cular load increases and becomes more static. A French research group showed in their classical study that immobility and musculoskeletal load increased when the rate of work increased (Teiger, Laville and Duraffourg 1974).
4. Posture is a source of information on the events taking place at work. Observing posture may be intentional or unconscious. Skilful supervisors and workers are known to use postural observations as indicators of the work process. Often, observing postural information is not conscious. For example, on an oil drilling derrick, postural cues have been used to communicate messages between team members during different phases of a task. This takes place under conditions where other means of communication are not possible.

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:

1. Self-reporting questionnaires and diaries. Self-reporting questionnaires and diaries are an economical means of collecting postural information. Self-reporting is based on the perception of the subject and usually deviates greatly from “objectively” observed postures, but may still convey important information about the tediousness of the work.
2. Observation of postures. The observation of postures includes the purely visual recording of the postures and their components as well as methods in which an interview completes the information. Computer support is usually available for these methods. Many methods are available for visual observations. The method may simply contain a catalogue of actions, including postures of the trunk and limbs (e.g., Keyserling 1986; Van der Beek, Van Gaalen and Frings-Dresen 1992) .The OWAS method proposes a structured scheme for the analysis, rating and evaluation of trunk and limb postures designed for field conditions (Karhu, Kansi and Kuorinka 1977). The recording and analysis method may contain notation schemes, some of them quite detailed (as with the posture targeting method, by Corlett and Bishop 1976), and they may provide a notation for the position of many anatomical elements for each element of the task (Drury 1987).
3. Computer-aided postural analyses. Computers have aided postural analyses in many ways. Portable computers and special programs allow easy recording and fast analysis of postures. Persson and Kilbom (1983) have developed the program VIRA for upper-limb study; Kerguelen (1986) has produced a complete recording and analysis package for work tasks; Kivi and Mattila (1991) have designed a computerized OWAS version for recording and analysis.

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.

1. The International Labour Organization published a Recommendation in 1967 on maximum loads to be handled. Although the Recommendation does not regulate postural elements as such, it has a significant bearing on postural strain. The Recommendation is now outdated but has served an important purpose in focusing attention on problems in manual material handling.
2. The NIOSH lifting guidelines (NIOSH 1981), as such, are not regulations either, but they have attained that status. The guidelines derive weight limits for loads using the location of the load—a postural element—as a basis.
3. In the International Organization for Standardization as well as in the European Community, ergonomics standards and directives exist which contain matter relating to postural elements (CEN 1990 and 1991).

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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:

• bones: levers, structural members

• flesh: volumes and masses

• joints: bearing surfaces and articulations

• joint linings: lubricants

• muscles: motors, springs

• nerves: feedback control mechanisms

• organs: power supplies

• tendons: ropes

• tissue: springs

• body cavities: balloons.

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:

1. Muscles come in pairs. Muscles can only contract, so for any joint there must be one muscle (or muscle group) to move it one way and a corresponding muscle (or muscle group) to move it in the opposite direction. Figure 1 illustrates the point for the elbow joint.
2. Muscles contract most efficiently when the muscle pair is in relaxed balance. The muscle acts most efficiently when it is in the midrange of the joint it flexes. This is so for two reasons: first, if the muscle tries to contract when it is shortened, it will pull against the elongated opposing muscle. Because the latter is stretched, it will apply an elastic counterforce that the contracting muscle must overcome. Figure 2 shows the way in which muscle force varies with muscle length.

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.

Applications

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

Seated posture

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.

Screwdriving

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:

1. How much can be handled without damage to the body (in the form, for example, of muscle strain, disc injury or joint problems)? This is called the biomechanical criterion.
2. How much can be handled without overexerting the lungs (breathing hard to the point of panting)? This is called the physiological criterion.
3. How much do people feel able to handle comfortably? This is called the psychophysical criterion.

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.

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 factors.

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

Where

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).

Lifting index.

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 posture in which the handling activity is performed

• the force exerted

• the direction of the force exertion

• the number of hands exerting the force

• the percentile of the population under study.

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.

Back

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.

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 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.

Figure 2. Diagrammatic presentation of the control of disposition to work by means of inhibiting and activating systems

Clinical fatigue

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.

Preventive Measures

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.

Back

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 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.

1. Physiological symptoms: fatigue is interpreted as a decrease of functions of organs or of the whole organism. It results in physiological reactions, e.g., in an increase of heart rate frequency or electrical muscle activity (Laurig 1970).
2. Behavioural symptoms: fatigue is interpreted mainly as a decrease of performance parameters. Examples are increasing errors when solving certain tasks, or an increasing variability of performance.
3. Psycho-physical symptoms: fatigue is interpreted as an increase of the feeling of exertion and deterioration of sensation, depending on the intensity, duration and composition of stress factors.

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

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:

with

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

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.

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