ABSTRACT
Some work-related psychosocial exposures can also induce activation of shoulder and neck muscles, especially cognitive loads (Lundberg et al., 1994; Westgaard and Bjørklund, 1987). Not all muscles are equally responsive to such exposures, but the upper trapezius muscle may respond with higher and more prolonged activation than most others (Wærsted and Westgaard, 1996). Interestingly, reaction times of eye movements decrease when the upper trapezius muscle is activated up to 30 per cent MVC (Kunita and Fujiwara, 1996). The muscular response to mental effort varies greatly between subjects and is partly dependent on emotional factors, e.g. a subject’s motivation (Wærsted et al., 1994). The relationship to colleagues and superiors may also have an indirect effect on muscle tension by a similar mechanism. Increased muscle activity due to mental loads serves no biomechanical purpose, since muscular forces are not required to perform mental operations. However, the peripheral mechanisms of muscle activation are probably not different from those that are due to physical exposures. As the activation of motor units in a muscle or at least within a motor neuron pool occurs in a predictable order, mental loads may cause prolonged or repeated activation of the same low-threshold motor units (Wærsted et al., 1996).
