Recruitment order of motoneurons during functional tasks.

The recruitment order of motoneurons of first dorsal interosseous (1DI) muscle has been shown to be the same during either isometric abduction or flexion of the index finger. However, it is not known whether this stereotyped recruitment is the same for all functional tasks. This question was addressed by using the 1DI muscle, which contributes to many functional tasks involving the hand and fingers. Single motor units and surface electromyographic activity were recorded from 1DI in three human subjects. Each subject performed tasks of abduction of the index finger, rotation to unscrew a threaded knob and pincer to close a spring-loaded clip. The same motor units were activated for each of the three tasks, providing no evidence for the existence of task groups. The order of recruitment of pairs of selected motor units was the same for each of the three tasks.