Supplement

Although many hypotheses have been suggested in the last few decades about possible underlying mechanisms for the development of nonspecific musculoskeletal symptoms in particular and (chronic) pain in general, definitive knowledge is still lacking. One wellknown potential mechanism at the muscular level is the Cinderella hypothesis, namely, that the lack of recovery from repeated recruitment of the same motor units is responsible for fatigue and complaints when the same movements and forces are asked for in a repetitive manner. Interested readers are referred to, for example, the reports of Hagberg & Wegman (8), Johansson & Sojka (17), Armstrong et al (9), Wright (18), Main & Watson (19),.