Nonlinear twitch torque summation by motor units activated at M-wave and H-reflex latencies.

We have suggested previously that motor units recruited reflexively contribute to torque produced during neuromuscular electrical stimulation (NMES), but this has not been tested directly. The current experiments were designed to quantify the contributions to twitch torque made by motor units recruited at M-wave and H-reflex latencies. The relationship between M-wave amplitude and torque was not linear. Rather, increases in M-wave amplitude caused the largest torque increases when M-waves were small. In addition, the torque contributions made by motor units recruited at M-wave and H-reflex latencies did not sum linearly, as changes in H-reflex amplitude only caused significant changes in torque when M-waves were small (<18% M(max)). This nonlinear summation of torque can be explained by the different latencies of twitches evoked by M-waves and H-reflexes. Large M-waves produce strong contractions at a short latency, possibly introducing slack into adjacent muscle fibers and reducing the ability of motor units recruited reflexively to generate torque. Muscle Nerve 40: 221-230, 2009.