PRHENSION SYNERGIES DURING SMOOTH CHANGES OF THE EXTERNAL LOAD OR TORQUE: TIME AND HISTORY EFFECT ON MULTI-DIGIT COORDINATION

Open Access
Author:
Sun, Yao
Graduate Program:
Kinesiology
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
May 24, 2011
Committee Members:
  • Mark Latash, Thesis Advisor
Keywords:
  • Synergy
  • Safety margin
  • Prehension
  • History effects
Abstract:
In this thesis, we explored how digit forces and indices of digit coordination depend on the history of getting to a particular set of task parameters during static prehension tasks. In our first study, the participants held in the right hand an instrumented handle with a light-weight container attached on top of the handle. At the beginning of each trial, the container could be empty, filled to the half with water (0.4 l) or filled to the top (0.8 l). The water was pumped in/out of the container at a constant, slow rate over 10 s. At the end of each trial, the participants always held a half-filled container that has just been filled (Empty-Half), emptied (Full-Half), or stayed half-filled throughout the trial (Half-Only). Indices of co-variation (synergy indices) of elemental variables (forces and moments of force produced by individual digits) stabilizing such performance variables as total normal force, total tangential force, and total moment of force were computed at two levels of an assumed control hierarchy. At the upper level, the task is shared between the thumb and virtual finger (an imagined digit with the mechanical action equal to that of the four fingers), while at the lower level, action of the virtual finger is shared among the actual four fingers. Filling or emptying the container led to a drop in the safety margin (extra gripping force over the slippage threshold) below the values observed in the Half-Only condition. Synergy indices at both levels of the hierarchy showed changes during the Full-Half and Empty-Half trials. These changes could be monotonic (typical of moment of force and normal force) or non-monotonic (typical of tangential force). For both normal and tangential forces, higher synergy indices at the higher level of the hierarchy corresponded to lower indices at the lower level. Significant differences across conditions were seen at the final steady-state showing that digit coordination during steady holding an object is history dependent. The observations support an earlier hypothesis on a trade-off between synergies at the two levels of a hierarchy. They also suggest that, when a change in task parameters is expected, the neural strategy may involve producing less stable (easier to change) actions. The results suggest that synergy indices may be highly sensitive to changes in a task variable and that effects of such changes persist after the changes are over. In the second study, we studied characteristics of digit action and their co-variation patterns across trials during static holding of an object while the external torque could change slowly and smoothly. The subjects held in the air an instrumented handle with an attachment that allowed a smooth change in the external torque over about 12 s; the load was always kept constant. Series of trials were performed with no changes in the external torque, and with slow motion of the load that required a smooth change of the effort from a non-zero pronation value to zero (PR-0) and from a non-zero supination value to zero (SU-0). The handle was kept vertical at all times. Similar to the previous study, indices of variance and co-variation of elemental variables (forces and moments of force produced by individual digits) stabilizing such performance variables as total normal force, total tangential force, and total moment of force were computed at the two levels of the assumed control hierarchy. Adjustments in the total moment of force were produced primarily with changes in the moment produced by the virtual finger, particularly by changes in its normal force. The normal force of the thumb at the final state (which was the same across conditions) was larger in the two conditions with changes in the external torque. The safety margin was significantly higher in the PR-0 condition, and it dropped with the decrease in the external torque. A co-contraction index was computed to reflect moment of force production by the fingers acting against the total moment produced by the virtual finger. It was higher for the SU-0 condition. Most variance indices dropped with a decrease in the external torque. The co-variation indices, however, remained unchanged over the trial duration. They showed signs of a trade-off between the two levels of the assumed hierarchy: Larger indices at the higher level corresponded to smaller indices at the lower level. This study and the previous one document several previously unknown features of prehensile tasks. The results show that characteristics of digit action and interaction in such tasks depend not only on the magnitudes of external constraints but on a variety of other factors including time changes in the constraints and their history.