Myo-electric fatigue and force failure from submaximal static elbow flexion sustained to exhaustion

Static contraction to the limit of endurance was performed at 40% and 10% of the maximal voluntary contraction (MVC). A group of 11 men (10 in their twenties, one aged 44) had the surface electromyogram of the brachioradialis and the biceps brachii (BB) muscles of the right arm (elbow angle 135 degrees) recorded. Endurance times were 113 (SD 28) s (40% MVC) and 51 (SD 19) min (10% MVC). Prolonged contraction changed the root mean square (rms) amplitude, the median frequency (fm), and the average muscle fibre conduction velocity (CV, measured by cross-correlation) as follows: 40% MVC: rms amplitude, increase of 150%-200%; fm, decrease of 55%-60%; CV, decrease of 25%-40%; and 10% MVC: rms amplitude, increase of 400%; fm, decrease of 20%; CV, decrease of 0%-10%. Since the CV of 10% MVC changed little or not at all, the large rms amplitude increases were attributed to motor unit (MU) recruitment, i.e. a massively intensified central motor command. The relative fm decreases of both contraction levels surpassed the CV slowing to an extent increasing with the relative contraction time; the additional fm lowering was in part assumed to reflect central nervous system mediated regulation of the time dispersion of MU firing, principally synchronization/grouping of MU action potentials (AP). Electrical stimulation of the BB muscle and the performance of 100% MVC test contractions found uniform relative force failures due to the performance of 40% and 10% MVC contractions. From variations in amplitude and conduction time of compound action potentials (CAP), it seemed unlikely that reduced muscle fibre excitability/AP propagation failure was underlying the force losses at exhaustion. Rather, the well preserved CAP after 10% MVC and the recovery CAP of 40% MVC indicated excitation-contraction failure caused by sustained voluntary contractions.