Behavior of the long-latency stretch reflex prior to voluntary movement.

Attempts to elicit stretch reflexes from the 'relaxed' human long thumb flexor were made at various times prior to the onset of a voluntary contraction of that muscle. When the muscle was fully relaxed, there were no short or long-latency responses at the rates of stretch employed (about 600 degrees/sec). Without complete relaxation there was a small long-latency response, which occasionally increased in amplitude as the time for voluntary activation approached. This augmentation seemed to be a result of a subtle parallel augmentation of background EMG, anticipating the major voluntary contraction, thereby increasing the gain of the long-latency stretch response.