Internal models of limb geometry in the control of hand compliance.

The aim of this article is to describe the role of some neural mechanisms in the adaptive control of limb compliance during preplanned mechanical interaction with objects. We studied the EMG responses and the kinematic responses evoked by pseudorandom perturbations continuously applied by means of a torque motor before and during a catching task. The temporal changes of these responses were studied by means of an identification technique for time-varying systems. We found a transient reversal of EMG stretch reflex responses centered on the time of ball impact on the hand; this reversal results in a transient coactivation of antagonist muscles at both the elbow and the wrist. The kinematic responses describe the relation between torque input and position output. Thus, they provide a global measure of limb compliance. The changes in limb compliance during catching were quantified by computing error criteria either in the Cartesian coordinates of the hand or in the angular coordinates of the elbow and wrist joints. We found that only the hand compliance in Cartesian coordinates is consistently minimized around impact, in coincidence with the transient reversal of the stretch reflex responses. By contrast, the error criteria expressed in the angular coordinates of the joints have a variable time course and are not minimized around impact. It is known that hand compliance depends on both the pattern of muscle activities and the geometrical configuration of the limb. Therefore, the lack of consistent correlation between the changes in hand compliance and the changes in the geometrical configuration of the limb during catching indicates that the gating of the stretch reflex responses around impact time is based on an internal model of limb geometry.