Towards understanding of human movements constrained by the external environment

The paper deals with the problem of the trajectory formation in human movements constrained by the external environment. To investigate this problem, we examine reaching movements of the human arm in a crank rotation task. The experimental data show that after learning and accommodation the position, velocity, and the tangential force trajectories converge to unique profiles while that of the normal force do not. To explain the formation of the human trajectories observed in the experiments, we resort to the optimization approach. Different criteria of optimality are analyzed using simplified kinematic and dynamic models. From the kinematic analysis we suggested that in the process of learning and adaptation the brain constructs a suitable parameterization of the constraint manifold. In the dynamic analysis it is found that the human trajectories cannot not be captured by a single conventional criterion. It is shown that a weighted combination of the hand force changes and the joint torque changes produces better results but brings the problem of weight selection.