The relationships between movement-related cortical potentials and motor unit activity during muscle contraction

This study investigated the relationship between movement-related cortical potentials (MRCP) and motor unit (MU) activity during muscle contractions. Ten right-handed, healthy males performed three motor tasks which were designed to bring about three different MU recruitment patterns while eliciting identical terminal force levels. In task I, the subjects were asked to perform 'shots' of self-paced isometric elbow flexion at 20% of maximal voluntary contraction (MVC). In task II, subjects performed 'holds' and maintained a constant isometric contraction for 2 s. In task III, task II was again performed while circulation was occluded prior to and during the trials. The arterial occlusion was induced by a pressure cuff around the upper-arm which was inflated to 200 mmHg for 5 min before the trials. Electroencephalogram (EEG) signals were recorded from scalp electrodes 2 s before and 2 s after the initiation of the motor tasks. The myoelectric (EMG) signals from the biceps brachii were recorded together with the force measurement. These simultaneously recorded EEG, EMG and force data were time-locked to the onset of force (1% of the target force) by an on-line computer system and then averaged for 50 trials for each subject. Grand average data indicated that there were no significant differences in the grand mean force levels exerted among the three tasks. The averaged EMG amplitude during force output in task III, however, was significantly (P < 0.001) greater than those in task II. The significantly greater EMG amplitude on the identical force output that was observed may thus be due to progressive recruitment of additional MU to compensate for the deficit in force development during trials with arterial occlusion. The averaged mean MRCP amplitudes after the onset of movement (AM potentials) were significantly (P < 0.001) greater in order of tasks III, II and I at all electrode locations (C3, Cz, C4). Our data suggest that the mean amplitude of AM potentials was related to MU activity, i.e., the higher the negativity of AM potentials, the greater the MU activity.