Changes in short and long latency stretch reflexes prior to movement initiation.

The amplitudes of both short (M1) and long (M2) latency stretch reflexes mechanically elicited in the triceps brachii muscle were investigated prior to its voluntary activitation. The results obtained from 14 healthy subjects indicated that the amplitude of the M2 response increased steadily as the perturbation was delivered closer to the movement suggesting that the M2 changes are associated with the preparation to respond to the visual stimulus. In contrast, the amplitude of the M1 response showed no systematic variations over the same time period. This dissociation of the M1 and M2 reflex responses prior to movement may be attributable to selective gain modulation in supraspinal stretch reflex pathways or to descending projections to polysynaptic spinal pathways.