Long‐latency stretch reflexes as co‐ordinated functional responses in man.

1. Reflex activity in human arm muscles has been measured in response to torque perturbations around the elbow joint in the flexion‐extension direction and/or the supination‐pronation direction. Intramuscular fine‐wire electrodes were used to record electromyographic (EMG) activity in the muscles. A pre‐load was applied in the same direction as or in a different direction to the perturbation. The subjects were instructed to ‘hold on’, which means that they had to actively resist the perturbation without unduly co‐activating their muscles. 2. The EMG activity showed segmented reflex responses with short‐latency (25‐50 ms) and long‐latency (50‐75 ms) components, followed by other bursts of activity which probably originated from the subject's voluntary or triggered reactions. 3. Motor units in m. triceps gave short‐latency and long‐latency responses to imposed elbow extension but only long‐latency responses were seen to imposed pronation, which does not stretch m. triceps. Motor units in m. brachialis gave short‐latency and long‐latency excitatory (inhibitory) responses to imposed extension (flexion) torques. However, only long‐latency inhibitory responses were observed to imposed pronation torques. Motor units in m. biceps gave short‐latency and long‐latency excitatory responses to imposed pronation torques. 4. These results show that the long‐latency reflex cannot be the result of a simple feed‐back mechanism that controls muscle length only. We argue that the long‐latency reflex activity reflects the co‐ordinated activation of muscles which is necessary for an adequate response. This hypothesis is supported by the fact that the distribution of long‐latency reflex activity over muscles was found to be similar to the distribution of activation found during a voluntary exerted torque in the direction opposite to the applied perturbation.