Motor-unit recruitment during long-term isometric and wrist motion contractions: a study concerning muscular pain development in computer operators

Work-related musculoskeletal disorders are frequent in workers with light to moderate load repetitive work, e.g. computer operators. The Cinderella hypothesis proposes that the development of chronic muscular pain is due to an overuse of fibers belonging to low-threshold motor units. The hypothesis requires that there exist motor units that are active (1) for a time long enough to actually damage the fibers, and (2) in different postures and throughout movements. In this study, the firing behavior of motor units in the extensor digitorum communis (EDC) muscle was registered in eight subjects during a 25-min static low-level contraction and during full range of motion wrist movements. The results showed one motor unit that was continuously active throughout the long-term measurement, while the majority of the motor units were partially active over time, i.e. it is possible for motor units in the EDC to be continuously active for at least 25 min of a low-level static contraction, but motor unit substitution seems to occur frequently. During the wrist motion, motor units were found active in all phases of the movements in all subjects. Hence, the results do not reject the Cinderella hypothesis for the EDC muscle.