Muscle membrane excitation and impulse propagation velocity are reduced during muscle fatigue

In order to determine whether or not impulse propagation was impaired during muscle fatigue, evoked muscle compound potentials (MCP) and twitches were recorded, both before and after fatigue, from the first dorsal interosseus (FDI), adductor pollicis (AP), and anterior tibialis (AT) muscles following supramaximal ulnar and peroneal nerve stimulation, respectively. The muscles were fatigued by maintaining maximum voluntary isometric, index finger abduction, thumb adduction, or ankle dorsiflexion for 1-5 minutes. FDI was most markedly altered, with reduced MCP amplitude (mean 32%) and increased MCP duration (mean 47%) after only 1 minute. After fatigue of longer duration (3-5 minutes), there were corresponding reductions in both the MCP amplitudes and the twitch tensions recorded from both the FDI and ankle dorsiflexors. We conclude that (1) a reduction in both the level of excitation and impulse propagation velocity of muscle membranes occurs during muscle fatigue, and (2) the magnitude of this reduced membrane function and its contribution to the mechanisms underlying fatigue depend both on the duration and degree of fatigue, as well as on the intrinsic properties of the particular muscle.