Effect of changes in the arm physical parameters on the minimum torque-change trajectories of human reaching movements

Abstract The minimum torque-change model is a computational model describing the trajectory formation of the point-to-point reaching movement in humans. This model roughly predicts a straight hand trajectory with a bell-shaped velocity profile, as observed in human reaching movements. However, the minimum torque-change criterion is a dynamic quantity, and the calculated trajectories could be, at least to some extent, affected by changes in the arm’s physical parameters such as mass, moment of inertia, and viscosity of each link. This study systematically investigates how changes in the arm’s physical parameters affect the optimal arm trajectories calculated based on the minimum torque-change criterion. The calculated optimal trajectories were largely curved, particularly when the physical parameters of the forearm were doubled or halved from the original physical parameters. Furthermore, when the original parameters were modified to be biomechanically more appropriate, the trajectories were also largely curved, unlike those in actual human reaching movements. The results suggest that the hand trajectory in human reaching movements may be determined by a dynamic optimization criterion that is less sensitive to variations in the biomechanical properties.