ABSTRACT
The application of electromyography (EMG) in ergonomics is motivated by the problem of musculoskeletal complaints in the work-place. It is commonly assumed that such complaints are mediated through biomechanical forces acting in the body, since awkward postures and repetitive movement are identified as risk factors in epidemiological studies (Winkel and Westgaard, 1992). However, biomechanical forces may also result in improved health (Åstrand, 1988) and a key issue is then to distinguish forces that are health promoting from those that cause impaired health. The chain of events postulated to result in health effects is summarized in Figure 8.1. Work demands (external exposure, i.e. physical demands quantified independently of the worker) exert their effects through muscle activity patterns generated to meet these demands (internal exposure). The muscle activity causes short-term responses, e.g. increased circulation, fatigue, and a variety of metabolic responses, that on a longer time scale may result in health effects. Factors independent of muscle activity influence the outcome and thus modulate the assumed relationship. Later in this paper evidence to suggest that muscle pain may arise independently of muscle activity is discussed, a mechanism not considered in Figure 8.1. In this model, a surface EMG recording may be used to estimate muscle activity (internal exposure) and thereby, hopefully, provide an estimate of the risk of a future development of a health problem. This scheme depends on the existence of a physical exposure-health effect relationship and an important research objective within ergonomics is to provide information about this relationship, if existing.
