Behavior of the stretch reflex in a multi-jointed limb.

When an external perturbation acts on a segment of a multi-jointed limb, angular motion results in all limb segments which are inertially linked to the one which is perturbed. In such a situation, different hypotheses on the functional role of the stretch reflex lead to different predictions on reflex responses by the stretched muscles. These predictions were examined experimentally and it was found that the EMG responses of elbow muscles evoked by forces applied to either the upper arm or the forearm were not uniquely related to changes in elbow angular position. They were instead correlated with the direction of changes in the net elbow torque resulting from the perturbation.