Human motor unit activity during the onset of muscle fatigue in submaximal isometric isotonic contraction.

The onset of fatigue has been studied by relating motor unit activity and surface myoelectric activity during constant force isometric contraction in man. The surface (global) EMG of the biceps brachii was recorded using bipolar electrodes. The global EMGs of the other elbow flexors were also recorded. Motor unit activity was recorded simultaneously at three points of the biceps brachii by using wire electrodes. The time course of the integrated surface EMG showed that fatigue occurred right at the beginning of the contraction. The increase of the integrated EMG was especially important when the value of the force maintenance was high. The time course of this increase was close to that of the output of the spikes recorded by the wire electrodes. No matter what the value of the force maintenance, MU recruitment occurred throughout the contraction. The existence of an increase in the MU firing frequently depended on the value of this force. Contractile element fatigue is believed to provide the explanation of these results, while neuromuscular junction fatigue may perhaps be more important near the time limit.