Quantitative evaluation of the electromyographic responses to multidirectional load perturbations of the human arm.

1. Force perturbations consisting of a random train of pulses were applied to the forearms of human subjects, the direction of the force being varied +/- 60 degrees from vertical in the sagittal plane in different trials. 2. Both forearm and upper arm were free to move, and the perturbations resulted in angular motion and torque at both joints. By varying the direction of the force, different combinations of these variables could be obtained. 3. Average angular motion and net torque at the shoulder and elbow joints and electromyographic activity of shoulder and elbow muscles due to a single pulse of force were computed by cross-correlation methods. 4. The pattern of responses in biceps, brachio-radialis, and anterior deltoid was not related uniquely to angular motion at the shoulder or elbow joints. Furthermore, the responses appeared to consist of two distinct components, an "early" one with a latency less than 40 ms and a "late" one with a latency of approximately 80 ms. 5. The average amplitude of the early response was best correlated with the average change in angular velocities, whereas that of the late one was best correlated with average changes in torque resulting from the perturbation. The data are consistent with the hypothesis that the two components have different anatomical substrates and that they have different functional implications for the stabilization of the limb in the face of perturbations.