Responses of mono- and bi-articular muscles to load perturbations of the human arm.

We studied the behavior of muscles acting synergistically in elbow flexion in response to load perturbations. The perturbations were applied either proximally or distally to the elbow joint and consisted of single pulses or steps of torque and of pseudo-random sequences of torque pulses. They produced changes in angular position and torque at both the shoulder and elbow joints. The electromyographic (EMG) responses evoked in biceps, brachio-radialis and brachialis muscles were different when elbow and shoulder motion was in the same direction and when the two angular motions were oppositely directed. For example, elbow extension resulted both when a downward force perturbation was applied to the forearm as well as when a posteriorly directed force applied to the upper arm was released. Elbow flexors were activated at a short latency only in the former case and not in the latter. The modulation of EMG activity in elbow flexors evoked by the perturbations was related to the global motion of the limb, including the angular motions at both the shoulder and elbow joints. The time course of the EMG responses in biceps, which acts on both joints, differed from that of brachio-radialis and brachialis muscles, which act only at the elbow. The results are discussed in the context of the possible mechanisms responsible for the muscle responses to the perturbations.