Human motor units studied by spike-triggered averaging and intraneural motor axon stimulation.

When low-threshold motor units are activated at low rates during sustained, weak voluntary contractions, most unit force profiles exhibit fatigue but some force potentiation. These data, obtained by spike-triggered averaging, are compared to the fatigue resistance of human motor units activated at twitch and tetanic rates by intraneural motor axon stimulation. With the latter technique, representative sampling of the motor units from one muscle group shows that unit force fatigue or potentiation at submaximal frequencies, and contractile rate changes, dictate the shifts in unit force-frequency relationships. More diverse fatigue protocols, and when possible, careful comparisons of data obtained by both these techniques, are needed to further our understanding of the force and frequency changes of single motor units during voluntary and stimulated exercise.