Optimal trajectory formation of human reaching movement in crank-rotation task

Opening a door, turning a steering wheel, rotating a coffee mill are typical examples of human movements constrained by the external environment. The constraints decrease mobility of the human arm and lead to redundancy in the distribution of interaction force between the arm joints. Due to the redundancy of force actuation in the constrained motions, there is an infinite number of ways to form the arm trajectory. However, humans form the hand trajectory in a unique way. How do humans resolve the redundancy of the constrained motions and specify the hand trajectory? To investigate these problems, we examine the trajectories of the human arm in a crank-rotation task. To explain the trajectory formation in constrained point-topoint motions, we propose a novel criterion minimizing hand contact force change and torque change over the time of the movement. Our experiments show a close matching between the prediction and the subjects' data. This indicates that in constrained movements the CNS naturally selects a moving contact frame as the criterion for the trajectory formation.