Discharges of single motor units at voluntary contraction of abductor digiti minimi muscle in man.

Isometric tension in a muscle can be graded by varying the firing rate of each motor unit as well as by varying the number of units which are recruited. Several important reports have been published dealing with the relation between the tension produced in a muscle and firing rate of single motor units in voluntary contraction in human muscles 2-4. However, the question of how the firing rate could be altered by varying the speed of contraction has not yet been pursued systematically, although it was reported that the integrated electrical activity increased linearly with contraction velocity 1. Furthermore, little information is available concerning what fraction of units are recruited for each stage of weak, moderate and strong contraction. Experiments are in progress at our laboratory to investigate this problem, and it is considered an important prerequisite for this investigation to observe if the amount of tension at which individual units are recruited is subject to considerable change by difference in speed of contraction. The aim of the present study is to observe discharges of single motor units during voluntary contraction, to learn how their firing rate is altered and to see whether they are recruited differently by varying speeds of contraction. For this purpose, it is necessary to record discrete discharges of single units along with tension changes produced in a muscle up to the full extent of its contraction. The experiments were performed on the abductor digiti minimi muscle of l0 human subjects whose ages ranged from 21 to 50 years. This muscle is the only muscle concerned in abduction movement of the little finger. Therefore, if movements at wrist and forearm are prevented by fixation (see below), the measured tension can be regarded as developing in the muscle. In order to record discrete discharges of single