Motor unit spike counts before and after maximal voluntary contraction

Motor unit spike counts in the biceps brachii muscle were evaluated using a monopolar needle electrode during maintenance of antigravity posture with the elbow flexed to 45 degrees before and after maximal voluntary contraction (MVC). After MVC, the number of motor unit spikes/second needed to maintain this posture was about 50% less than prior to MVC. Surface rectified integrated EMG activity declined in parallel to the decline in spike counts. To determine whether different muscles were compensating for the reduction in spike counts, synergistic muscles were examined simultaneously. Similar reductions were noted. In one deafferented subject, EMG activity increased rather than decreased after MVC . Fatigue decreases the contraction-relaxation rate of muscle fibers, which lowers fusion frequency. Thus, lower rates of motor unit activation can result in the maintenance of constant force. A feedback system from muscle to CNS likely senses this slowing and leads to the spike count reduction.