BEHAVIOR OF MOTOR UNITS IN HUMAN BICEPS BRACHII DURING A SUBMAXIMAL FATIGUING CONTRACTION

The activity of 50 single motor units was recorded in the biceps brach ii muscle of human subjects while they performed submaximal isometric elbow flexion contractions that were sustained to induce fatigue. The purposes of this study were to examine the influence of fatigue on mot or unit threshold force and to determine the relationship between the threshold force of recruitment and the initial interimpulse interval o n the discharge rates of single motor units during a fatiguing contrac tion. The discharge rate of most motor units that were active from the beginning of the contraction declined during the fatiguing contractio n, whereas the discharge rates of most newly recruited units were eith er constant or increased slightly. The absolute threshold forces of re cruitment and derecruitment decreased, and the variability of interimp ulse intervals increased after the fatigue task. The change in motor u nit discharge rate during the fatigue task was related to the initial rate, but the direction of the change in discharge rate could not be p redicted from the threshold force of recruitment or the variability in the interimpulse intervals. The discharge rate of most motor units de clined despite an increase in the excitatory drive to the motoneuron p ool during the fatigue task.