The nature of suprasegmental influences on the late reflex activity in human forearm muscles

The earliest component of the late EMG response (M2) to sudden stretch of an isometrically contracting muscle is influenced by suprasegmental mechanism, but whether these are tonic, phasic, or both, is uncertain. We have therefore investigated the nature of these mechanisms by varying the predictability of the direction of perturbation of forearm muscles held isometrically against a constant flexor or extensor force of 2.3 N. We found that the M2 response did not change appreciably regardless of whether the direction of perturbation was known in advance or which hand was used by the subject. These findings suggest that any tonic supraspinal influence of the M2 response is of little significance compared to the importance of phasic mechanisms and provide support for the existence of a cerebral arc subserving this reflex component.