Myofeedback system

Work related musculoskeletal disorders (WMSD) have become an increasing problem for western industrialised countries. For example in the Netherlands, 254.780 of the 859.000 in 1997 disabled people have musculoskeletal complaints and one of the five sickdays is related to these disorders (Voskamp, 1998). A subset of WMSD is myalgia, which is most common in the shoulder/neck region. This type of myalgia is related to low level static workload (Hagberg et al., 1995). Such load situations become more and more common in the working life of today.

Traditionally, it has been expected that a reduction of the static load level would prevent the development of WMSD especially myalgia. However literature shows that even when the muscle force is very low, subjects develop WMSD. This makes obvious that there is no acceptable limit for static load, no matter how low (Westgaard, 1988).

Ergonomic studies, mostly performed in the Scandinavian countries, show that surface EMG (SEMG) is one of the most important tools to get insight in the physical exposure and its physiological consequences. Hägg (1991) presented "the Cinderella theory" as a mechanism related to the development of myalgia in the shoulder/neck region. This theory suggests that specific type I motor units are recruited permanently during sustained manual work and that short time intervals (gaps) with a totally relaxed muscle are essential to relieve these low threshold motor units (Hägg, 1991). Veiersted et al (1993) showed in a longitudinal study that the lack of short moments with total muscle relaxation (EMG-gaps) predicts later myalgia. During the past few years, there is increasing evidence for this Cinderella theory (Veiersted et al, 1994; Hägg and Åström, 1997). These gaps can only be detected with adequate surface EMG techniques.