Subject instruction and long latency reflex responses to muscle stretch.

1. Surface electromyographic recordings were made in eight normal subjects from the isometrically contracting elbow flexors before and during forcible extension of the elbow through 7 degrees in 50-150 msec. 2. Weh the subjects were instructed prior (2-5 sec) to a forthcoming stretch to 'resist' or to 'let go', they could reliably enhance or suppress e.m.g. activity occurring between 40 and 70 msec from commencement of the stretch. Such e.m.g. 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, however, none of them could adjust the long-latency reflex appropriately. 3. It is concluded that central, evaluative processes commencing at the time of a perturbation cannot influence long-latency reflex responses to that perturbation.