7

Work Capacity, Stress and Fatigue

The term (fatigue) should be absolutely banned from scientific discussion. Is a fatigue test possible? (B. Muscio (1921) British Journal of Psychology, 12:31-46)

Stress and fatigue are words used in a variety of ways by ergonomists. Sometimes they are used to refer to temporary states of parts of the body. At other times, they are used to refer to chronic states of the whole person. In both senses, stress, arising out of work demands, causes fatigue when work capacity is exceeded.


Stress and fatigue

Stress

Stress is often thought of in a negative way in everyday life. In technical discussion, however, it is perhaps more appropriately viewed as 'applied load' or 'task demand'. The classical physical model of stress is based on Hooke's law: if we place a load of 30 kg on horizontal beam and the beam bends under the load, the stress is 30 kg and the strain is the deformation of the beam. Once the stress is removed, the beam will resume its former shape. Under high loads, the elastic limit of the beam will be exceeded and the beam will not resume its original shape when the stress is removed. Permanent damage results because the stress exceeds the load-bearing capacity.

The classical physiological model of stress is derived from the work of H. Selye espoused in his book The Stress of Life (Selye, 1956). Selye was interested in the endocrinological responses to life events and his key insight was that many, very different, noxious stimuli produce the same effects. This lead Selye to coin the term 'general adaptation syndrome' - a three-phase response to stress consisting of an alarm reaction when the threat is perceived, then resistance, followed by adaptation and finally exhaustion or death. To this day, researchers continue to use endocrine markers such as urinary catecholamine concentration and salivary cortisol levels to assess the overall level of work or life stress. It is believed that the former indicates the level of stress and resulting physiological arousal, whereas the latter indicates the degree of emotional response to the situation in which the stress is experienced (Lundberg, 1995). Salivary cortisol levels, taken early in the morning are used to assess recovery from stress - higher levels indicate delayed recovery, from which higher levels of work stress are inferred. Sluiter et al. (2000), for example, found raised levels of cortisol and adrenaline in workers with jobs that combined mental

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