An understanding of how the brain constrains dimensions of freedom to control the body would be beneficial for the robotic engineering of a humanoid robot. We estimated joint stiffness in a female Japanese monkey (Macaca fuscata) during arm reaching movements and carried out a numerical simulation. The estimated stiffness was high at movement onset and movement end, and decreased at the mid-point of the movement. These characteristic patterns were reproduced by the numerical simulation using a 2-link 6-muscle arm model and an approximately optimal feedback control. Although the arm model was a redundant system with multiple dimensions of freedom, the optimal control was able to solve the redundancy problem by optimizing a task-relevant cost function. We suggest that the brain may control the body according to a similar optimal control law.
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