The control of rotational hand actions by multi-digit synergies

The current dissertation is the first to systematically investigate finger coordination during multi-finger tasks that require accurate rotational actions. This series of studies has five main objectives: to quantify multi-digit synergies during actions with an explicit rotational component; to test the hypothesis that multi-digit synergies allow the stabilization of both total force (FTOT) and total moment of force (MTOT); to explore the effect of healthy ageing on multi-finger synergies stabilizing FTOT and MTOT; to describe the variance structure of elemental variables (finger modes) with respect to stabilization of FTOT or MTOT; and to examine hierarchies of multi-digit synergies during rotational actions performed with a hand-held object. A series of experiments within a range of tasks including isometric four-finger pressing tasks and free rotation of a hand-held object in 3D space resulted in several main conclusions. First, young subjects showed a strong moment-stabilizing synergy but not a force-stabilizing synergy in both stationary and non-stationary accurate moment-production tasks. The presence of multi-finger synergies stabilizing MTOT could be viewed as a default conditioned by everyday tasks. Second, flexible combinations of commands to fingers were found that satisfied both resultant force and moment of force constraints. Third, adding a secondary moment constraint did not interfere with the force stabilizing synergy. Two co-existing synergies stabilizing two performance variables simultaneously without an interaction confirmed the principle of superposition. These results emphasize a major advantage of using multi-finger synergies as compared to finding unique optimal solutions, namely the possibility to perform secondary tasks without detrimental effects on a primary task. During rotation of the hand-held object, the negative co-variation among elemental variables (forces produced by individual digits) stabilizing their combined output was observed at the hierarchically higher level (virtual finger and thumb level), but not at the lower level (individual finger level). At both levels, elemental moments of force negatively co-varied across trials, only during steady-states and this co-variation disappeared during the object rotation. These findings suggest that synergies at two levels of a control hierarchy can co-exist but that there is also a trade-off between synergies at the two levels. Finally, elderly persons showed significantly lower indices of synergies stabilizing the moment of force as compared to the young persons. These age-related differences might be causally related to the documented impairment of the hand function with age. This research line contributes to the current understanding of the hand control in healthy persons, the impaired hand function in the elderly, and has implications for hand rehabilitation following neurological disorders, trauma, or healthy aging.