Contributions of single and double joint stiffness of human arm during force control

To investigate the motion control mechanism of human arm during force control, shoulder, elbow, and double-joint stiffness were measured by applying a small perturbation, and their contributions to joint torques were estimated. Each joint stiffness greatly altered for the different force direction at hand, and shoulder and elbow single joint stiffness were linearly correlated to each joint torques. By assuming a linear joint stiffness, generated torques were decomposed into torques produced by each stiffness component. This analysis has revealed that the elbow torque was produced complementary by the single and double-joint stiffness, and that the shoulder torque was mainly produced by the shoulder single joint stiffness. Additionally, the double-joint stiffness was linearly correlated to the decomposed elbow joint torques produced by double-joint stiffness. These results suggest that shoulder and elbow torques are separately controlled by different muscles in this task.