Title:
Multi-digit coordination during lifting a horizontally oriented object: synergies control with referent configurations.

Abstract:
We explored digit coordination during the acceleration phase of a quick lifting movement of a hand-held horizontal object. We tested three hypotheses related to: (1) the scaling of mechanical variables produced by the hand with changes in the external load, torque, and moment of inertia; (2) changes in the safety margin for the thumb with both the loading conditions and acceleration; and (3) changes in the indices of synergies. The subjects held a horizontal handle with a prismatic grasp (the thumb acted on top of the handle) and performed series of "very quick" lifting movements to a visual target. Multi-digit synergies were quantified as co-variation indices among elemental variables (forces and moments produced by individual digits). The resultant force scaled with the external load but not torque, while the grip force scaled with the external torque but not load. The safety margin dropped with an increase in acceleration; it also showed changes with the external torque and moment of inertia. Total moment of force was primarily produced by the tangential forces (over 80 %) across all movement phases and loading conditions. The index and little fingers produced close to zero moment with their normal forces, while the middle and ring fingers produced consistent moments due to the reproducible shifts of their centers of pressure. Synergy indices at the upper level of the assumed hierarchy (the task is shared between the thumb and virtual finger--an imagined digit with the action equal to that of the four fingers combined) did not drop with acceleration for the three force vector components and one of the moment vector components. They did drop with acceleration at the lower level (virtual finger action is shared among the four fingers). There was a trade-off between synergy indices computed at the two levels for the three force vector components, but not for the moment of force components. We confirmed specialization of different fingers with respect to different task components in quick manipulation tasks. The findings have implications for hypotheses on the control of voluntary movements involving redundant sets of effectors. Within the referent configuration hypothesis, components of a referent configuration may be adjusted to task mechanical characteristics using simple scaling rules. The neural organization of multi-digit synergies in a hierarchal system is able to selectively protect synergies related to stabilization of some performance variables from detrimental effects of the rate of change of those variables. A large number of apparently redundant elemental variables are not the source of additional computational problems but may be beneficial, allowing the central nervous system to facilitate synergies at both levels of the hierarchy.