A new optimal control model for reproducing two-point reaching movements of human three-joint arm with wrist joint's freezing mechanism

A new optimal control model of human arms has been developed to simulate two-point reaching movement characteristics for human three-joint arms (shoulder, elbow, and wrist joints) and its fundamental performance has been clarified. The new model is formulated by extending the previous two-joint modified minimum torque-change model to a three-joint model with a freezing mechanism in its wrist joint and incorporating three kinds of energy costs into the previous model's evaluation function. The freezing mechanism is modeled as a feedback controller to simulate the muscle dynamics contracting agonists and antagonists together. Consequently, it was clarified that the wrist joint's freezing mechanism and the minimization of the moment power of joints or the energy consumed by viscosity resistance were important to reproduce two-point reaching movement characteristics for human three-joint arms. This suggests that the new model with the freezing mechanism in its wrist joint functions effectively as a model of the human three-joint arm control mechanism and that the energy optimization can involve the trajectory planning of two-point reaching movements.