Subject instruction and long latency reflex responses to muscle stretch.

Publisher Website
Google Scholar
Abstract
Surface electromyographic [emg] recordings were made in 8 normal subjects from the isometrically contracting elbow flexors before and during forcible extension of the elbow through 7.degree. in 50-150 ms. When the subjects were instructed prior (2-5 s) to a forthcoming stretch to ''resist'' or to ''let go'', they could reliably enhance or suppress emg activity occurring between 40 and 70 ms from commencement of the stretch. Such emg activity represents a ''long-latency'' (or ''M2'') reflex response: it occurs with a latency longer than the spinal segmental monosynaptic reflex, but shorter than a voluntary reaction time. When the subjects were given their instructions (by means of a light) at the moment the stretch commenced, none of them could adjust the long-latency reflex appropriately. Central, evaluative processes commencing at the time of a perturbation cannot influence long-latency reflex responses to that perturbation.