Principle of superposition in human prehension

The principle of superposition introduced by Prof. S. Arimoto and his colleagues for the control of robotic hand has been shown to be applicable to the control of prehensile actions by humans. In particular, experiments have shown that static human hand actions can be viewed as a superposition of two independent synergies controlling the grasping force and the orientation of the object. Studies of elderly persons have shown that they are impaired in both synergies and show worse stabilization of the grasping force and of the total moment of forces applied by the digits to a hand-held object. Recent studies have also shown that the principle of superposition holds with respect to reactions to expected and unexpected mechanical perturbations applied to a hand-held object. Indices of the two synergies have shown different changes following a perturbation. Generalization of the principle of superposition to human prehension is an important step towards understanding the principles of control of the human hand.