Does torque minimization yield a stable human grasp?

In this chapter, we present a study of the human grasp conducted with a mathematical formalism that has been developed in robotics during the last two decades. The main objective of this study is to assess the extent to which the structure of the hand is adapted to its grasping function by identify the conditions under which a model minimizing energy expenditure predicts a stable grasp. The idea is that the human hand, as the result of evolutionary pressure, must be designed in such a way that it can grasp objects with minimum effort. To test this hypothesis, we defined a cost function minimizing the weighted norm of the joint torque vector of a simple biomechanical model of the hand. The contact forces predicted by the model are then compared to the ones observed in a experimental study of the human tripod grasp. The results indicate that this cost function can predict a stable grasp when the external force is zero. A possible interpretation of this result is that the external force represents an unknown that cannot be taken into account by an evolutionary process. “The coordination of a movement is the process of mastering redundant degrees of freedom of the moving organ, in other words it s conversion to a controllable system” (Bernstein, [25], p. 127).