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.