![]() ![]() 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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