Dynamic properties of human motor units in the Hoffmann--reflex and M response.

The dynamic properties of motor units involved in unrestrained H-reflexes and M responses have been studied in seven normal adult males. A linear relationship was observed between the peak to peak EMG of the M response and its twitch tension. Linear relationships were also noted between the EMG and peak displacement, velocity, and acceleration of the evoked muscle twitch for both the H-reflex and the M response. Contraction times for motor units recruited in the H-reflex were longer than those for the M response at low levels of motor unit recruitment. For both responses, however, the contraction times (time to peak force) were markedly shorter than those reported for isometric conditions. Differences were also noted between the H-reflex and M response in the absolute values of the tension to EMG ratio. These differences, together with the longer contraction times for the H-reflex, are quite consistent with the view that different motor unit subpopulations are recruited in the early stages of the two responses. The results and their interpretation, in terms of differential motor unit recruitment, generally confirm the conclusions of studies in which isometric twitch contractions have been investigated.