Prehension synergies: principle of superposition and hierarchical organization in circular object prehension.

This study tests the following hypotheses in multi-digit circular object prehension: the principle of superposition (i.e., a complex action can be decomposed into independently controlled sub-actions) and the hierarchical organization (i.e., individual fingers at the lower level are coordinated to generate a desired task-specific outcome of the virtual finger at the higher level). Subjects performed 25 trials while statically holding a circular handle instrumented with five six-component force/moment sensors under seven external torque conditions. We performed a principal component (PC) analysis on forces and moments of the thumb and virtual finger (VF: an imagined finger producing the same mechanical effects of all finger forces and moments combined) to test the applicability of the principle of superposition in a circular object prehension. The synergy indices, measuring synergic actions of the individual finger (IF) moments for the stabilization of the VF moment, were calculated to test the hierarchical organization. Mixed-effect ANOVAs were used to test the dependent variable differences for different external torque conditions and different fingers at the VF and IF levels. The PC analysis showed that the elemental variables were decoupled into two groups: one group related to grasping stability control (normal force control) and the other group associated with rotational equilibrium control (tangential force control), which supports the principle of superposition. The synergy indices were always positive, suggesting error compensations between IF moments for the VF moment stabilization, which confirms the hierarchical organization of multi-digit prehension.