The posture that a person adopts when performing a particular task is determined by the relationship between the dimensions of the person’s body and the dimensions of the various items in his or her workspace (a tall person using a standard kitchen will stoop more than a short one, etc.). The extent to which posture is constrained in this way is dependent upon the number and nature of the connections between the person and the workspace. These connections may be either physical (seat, worktop, etc.) or visual (location of displays, etc.). If the dimensional match is inappropriate the short- and long-term consequences for the well-being of the person may be severe.
Posture may be defined as the relative orientation of the parts of the body in space. To maintain such an orientation over a period of time, muscles must be used to counteract any external forces acting upon the body (or in some minority of cases internal tensions within the body). The most ubiquitous of these external forces is gravity. Consider a standing person who leans forwards from the waist.
The postural loadings on the hip extensor or the back extensor muscles are proportional to the horizontal distance between the hip and lumbosacral joints, respectively, and the centre of gravity of the upper part of the body (i.e. the head, arms and trunk). The further the trunk is inclined the greater this distance becomes (Figure 3.10). Physiologists call the muscular activity that results from this loading
‘static work’.
Muscle as a tissue responds badly to prolonged static mechanical loading. (The same is probably true of other soft tissues, and even perhaps of bone, but the physiology of these cases is much less well understood.) Static effort restricts the flow of blood to the muscle. The chemical balance within the muscle is disturbed, metabolic waste products accumulate and the condition of ‘muscular fatigue’
supervenes. The person experiences a discomfort which is at first vague but which subsequently develops into a nagging pain until it becomes a matter of some urgency that relief is sought by a change of position. Should you require evidence of this course of events, you should raise one of your arms and hold it out in front of you as you continue to read (or attempt to do so). Provided our workspace and/or working schedule allows us to make the frequent shifts of posture which are subjectively desirable, all will be well—since the physiological processes of muscular fatigue are relatively rapidly reversible by rest or change of activity (particularly if the activity involves stretching the fatigued muscle).
In general, we may think of ‘fidgeting’ as our bodies’ defence against postural stress. This mechanism characteristically operates at a subconscious level—usually we fidget before we become consciously aware of discomfort. In relaxed sitting the sensory stimuli probably come more from the compression of the soft tissues of the buttocks and thighs than from muscle tension. The crossing and uncrossing of the
legs is a characteristic way of redistributing the pressure on the buttocks and, hence, pumping blood through the tissues. The rate of fidgeting can be used as an index of the comfort of chairs—the less comfortable we are, the more we fidget. It is a matter of common experience that other factors are involved. Some people fidget more than others and we all fidget more when we are bored—presumably because mental activity can ‘shut out’ the sensory stimuli that cause the fidgeting (or raise our threshold of discomfort). Such a hypothesis is in line with contemporary theories of the nature of pain (Melzack and Wall 1982). Students almost universally consider lecture theatre seating to be uncomfortable—is this to do with the seats or the lectures?
Physiologically, comfort is the absence of discomfort—I know of no nerve endings capable of transmitting a positive sensation of comfort from a chair. Comfort is a state of mind which results from the absence of unpleasant bodily sensations. (The same relationship does not hold, however, for pleasure and pain.) We shall consider the matter of sitting comfort at greater length in the next chapter.
Suppose that the working circumstances are such as to closely constrain us to a particular posture and prevent postural change—the consequences may be divided into those occurring over the short term and those occurring over the long term. In the short term, mounting discomfort may distract the operator from his task leading to an increased error rate, reduced output, accidents, etc. From the physiological standpoint, however, we are still talking about a reversible state—since the symptoms are relieved by rest or by a change of activity. At some point, nevertheless (and this point is not well defined since the transition is probably gradual rather than sharp), pathological changes in the muscle or soft tissue take over. Typically, pain
Figure 3.10 Biomechanical analysis of postural stress in a forward leaning position. Note that this analysis ignores the direct effect of the weight of the trunk which the spine must support even when dw=0. w is weight of that part of the body above the lumbo-sacral joint; c is the compressive force acting along the axis of the spine; t is the tension in the back muscles (erector spinae).
comes on after increasingly short periods of postural loading and rest is less certain to bring relief. At this point we are dealing not with discomfort but with physical injury and a disease process.
Back pain, neck pain and the class of conditions affecting the hand, wrist and arm which we refer to as work-related upper limb disorders (WRULD) or repetitive strain injuries (RSI) are all conditions that characteristically result from over-use of the muscles and other soft tissues in question. This over-use may be due to prolonged static loading, repetitive motions, acute over-exertion or some combination of these.
Psychological factors may also be involved (probably because psychological stress leads to increased muscle tension). We shall return to these matters in Chapter 8.
In general, a varied working posture is better than a fixed working posture; but if circumstances demand that you work in a fixed position (as in practice will very often be the case), then the deleterious effects that ensue will increase with the degree of static work required to maintain the position concerned. The following simple guidelines are based in part upon Corlett (1983); for a more detailed discussion see Pheasant (1991a).
Figure 3.11 The sewing machinist, from an original kindly supplied by Murray Sinclair. (From S.
Pheasant, Ergonomics, Work and Health, Macmillan, 1991, fig. 1.6, p. 12, reproduced with kind permission.)
(i) Encourage frequent changes of posture
Sedentary workers, therefore, should be able to sit in a variety of positions—some office chairs are now being designed with this in mind. For many industrial tasks a sit-stand workstation is to be advocated. The task is typically set at a height that is suitable for a standing person (see Section 3.8) and a high stool or ‘perch’ is provided as an alternative. There seems little doubt that most sedentary workers would be better off if their jobs required them to get up and move around once in a while.
(ii) Avoid forward inclination of the head and trunk (Figure 3.11)
This commonly results from visual tasks, machine controls or working surfaces that are too low (see below).
(iii) Avoid causing the upper limbs to be held in a raised position (Figure 3.12) This commonly results from a working level that is too high (or a seat that is too low). If manipulative tasks must be performed in a raised position, perhaps for visual reasons, arm supports should be provided. In addition to being a considerable stress to the shoulder muscles, tasks that must be performed at above the level of the heart impose an additional circulatory burden. The upper limit for manipulative tasks should be around halfway between elbow and shoulder level.
Figure 3.12 Deviated wrist positions in repetitive industrial tasks, showing movements of radial and ulnar deviation with an extended wrist in a packing task, where the working level is too high.
Note also the abduction of the shoulders. From an original kindly supplied by Peter Buckle. (From S. Pheasant, Ergonomics, Work and Health, Macmillan, 1991, fig. 14.1, p. 262, reproduced with kind permission.)
(iv) Avoid twisted and asymmetrical positions
These commonly result from expecting an operator to have eyes in the back of his head, i.e. from the mislocation of displays and controls.
(v) Avoid postures that require a joint to be used for long periods at the limit of its range of motion
This is particularly important for the forearm and wrist.
(vi) Provide adequate back support in all seats
It may be that for operational reasons the backrest cannot be used during the performance of the work task—but it will still be important in the rest pauses.
(vii) Where muscular force must be exerted the limbs should be in a position of greatest strength
Unless by so doing one of the foregoing rules is broken (see Section 3.9).