Observations on the time course of the electromyographic response reflexly elicited by muscle vibration in man.

Surface electromyography has been used to study the initial reflex response of various muscles to vibration, applied to their tendons, when the subject was already contracting them voluntarily. The response at the onset of vibration was of a latency appropriate for Ia monosynaptic action and was always highly phasic with an initial wave rising far above any maintained increase in electromyogram (e.m.g.) activity; its duration was typically well below 20 ms in the rectified average. Thus, there is nothing peculiar, in this respect, about flexor pollicis longus for which such behaviour has already been described, and used to draw certain wide-ranging conclusions about the stretch reflex. Theoretical considerations, developed in an Appendix, show that quite apart from the operation of any inhibitory mechanisms such a phasic response is to be expected from a population of tonically discharging motoneurones when there is a step increase in the level of their excitatory drive.