Muscle fatigue assessment during sustained high isometric contractions

The myoelectrical fatigue manifestation was assessed during sustained isometric maximal and submaximal (75% of maximal (MVC)) isometric contractions by means of a level-trigger averaging of the surface electromyogram (EMG) recorded from m. biceps brachii by branched bipolar electrodes. The conduction velocity (CV) of excitation was calculated from the time shift of the negative peaks of the averaged potentials (APs). The amplitude and the duration of the negative phase of APs were also measured. The conduction velocity was the most appropriate EMG-derived parameter for muscle fatigue assessment. The duration of the endurance time and the time pattern of CV changes during submaximal sustained contractions were found to resemble very closely those during maximal contraction efforts sustained to 30 percent reduction of the initial torque. Thus the recruitment of additional motor units (MUs) during sustained high contractions does not affect essentially the calculated values of CV.