![]() Prior to the lifting action, high synergy indices were observed at the individual finger level while modest indices were observed at the thumb-VF level. Multi-digit synergies were quantified using indices of co-variation between digit forces and moments of force across unperturbed trials. The aperture showed non-monotonic changes with a large, fast decrease and further increase, ending up with a smaller distance between the thumb and the fingers as compared to unperturbed trials. In such trials, the hand stopped at a higher vertical position and rotated into pronation or supination depending on the expected torque. In a few trials, the handle was unexpectedly fixed to the table and the digits slipped off the sensors. A load could be attached off-center to provide a pronation or supination torque. ![]() The subjects performed very quick vertical movements of a handle into a target. ![]() ![]() Synergies were analyzed at the thumb-virtual finger (VF) level (VF is an imaginary digit with the mechanical action equivalent to that of the four actual fingers) and at the individual finger level. We used the framework of the equilibrium-point hypothesis (in its updated form based on the notion of referent configuration) to investigate the multi-digit synergies at two levels of a hypothetical hierarchy involved in prehensile actions. ![]()
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