Indexed on: 14 Jul '17Published on: 14 Jul '17Published in: Journal of neurophysiology
Moving the arm is complicated by mechanical interactions that arise between limb segments. Such intersegmental dynamics cause torques applied at one joint to produce movement at multiple joints. Here, we investigated whether the nervous system accounts for intersegmental limb dynamics across the shoulder, elbow, and wrist joints during self-initiated planar reaching and when countering external mechanical perturbations. Our first experiment tested whether the timing and amplitude of shoulder muscle activity accounts for interaction torques produced during single-joint elbow movements from different elbow initial orientations and over a range of movement speeds. We found that shoulder muscle activity preceded movement onset and elbow agonist activity, and was scaled to compensate for the magnitude of interaction torques caused by forearm rotation. Our second experiment tested whether elbow muscles compensate for interaction torques introduced by single-joint wrist movements. We found that elbow muscle activity preceded movement onset and wrist agonist muscle activity, and thus the nervous system predicted interaction torques arising because of hand rotation. Our final experiments tested whether shoulder muscles compensate for interaction torques introduced by different hand orientations during self-initiated elbow movements and when countering mechanical perturbations that caused pure elbow motion. We found that the nervous system predicted the amplitude and direction of interaction torques, scaling shoulder muscle activity during self-initiated elbow movements and rapid feedback control. Our results demonstrate that the nervous system robustly accounts for intersegmental dynamics, and that the process is similar across proximal and distal arm muscles and between feedforward (self-initiated) and feedback (reflexive) control.