FINGER INTERACTION IN A THREE DIMENSIONAL PRESSING TASK: CONTROL OF FORCE MAGNITUDE AND DIRECTION AND AGE RELATED DIFFERENCES
Open Access
- Author:
- Kapur, Shweta
- Graduate Program:
- Kinesiology
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- March 30, 2010
- Committee Members:
- Mark Latash, Thesis Advisor/Co-Advisor
Mark Latash, Thesis Advisor/Co-Advisor - Keywords:
- finger interaction
aging
synergy
enslaving
hand
Motor Control - Abstract:
- This thesis addresses the indices of finger interaction in single- and multi-finger pressing tasks in different directions and the effects of aging on these interactions. Accurate control of forces produced by the fingers is essential for performing object manipulation. The first study presented in this thesis examines the indices of finger interaction when accurate time profiles of force are produced in different directions, while using one of the fingers or all four fingers of the hand. We hypothesized that patterns of unintended force production among shear force components may involve features not observed in the earlier studies of vertical force production. In particular, we expected to see unintended forces generated by non-task fingers not in the direction on the instructed force but in the opposite direction, as well as substantial force production in directions orthogonal to the instructed direction. We also tested a hypothesis that multi-finger synergies, quantified using the framework of the uncontrolled manifold hypothesis, will help reduce across-trials variance of both total force magnitude and direction. Young, healthy subjects were required to produce accurate ramps of force in five different directions by pressing on force sensors with the fingers of the right (dominant) hand. The index finger induced the smallest unintended forces in non-task fingers. The little finger showed the smallest unintended forces when it was a non-task finger. Task fingers showed substantial force production in directions orthogonal to the intended force direction. During four-finger tasks, individual force vectors typically pointed off the task direction, with these deviations nearly perfectly matched to produce a resultant force in the task direction. Multi-finger synergy indices reflected strong co-variation in the space of finger modes (commands to fingers) that reduced variability of the total force magnitude and direction across trials. The synergy indices increased in magnitude over the first 30% of the trial time and then stayed at a nearly constant level. The synergy index for stabilization of total force magnitude was higher for shear force components as compared to the downward pressing force component. The results suggest complex interactions between enslaving and synergic force adjustments, possibly reflecting the experience with everyday prehensile tasks. For the first time, the data document multi-finger synergies stabilizing both shear force magnitude and force vector direction. These synergies may play a major role in stabilizing the hand action during object manipulation. In the second study done on elderly participants, we looked at how the indices of finger interaction change with aging. We hypothesized that elderly participants will show lower enslaving not only for normal forces but also for shear forces. We expected the elderly to be less accurate in both force magnitude and direction. We expected the higher motor variability in the elderly to be associated with lower indices of synergies stabilizing those variables. These hypotheses have been confirmed. In addition, we found that, compared to younger participants, the elderly participants produced larger forces in directions orthogonal to the instructed force direction. This finding contrasts the lower indices of unintended force production by non-instructed fingers in the elderly. The time profiles of the synergy indices computed for both force magnitude and direction were similar across the younger and elderly groups: Both showed stronger synergies later in the trial. Quantitatively, however, these indices were significantly smaller in the elderly participants. This study has shown that aging is associated not only with lower voluntary force magnitude and lower accuracy of force production but also with changes in finger interaction. There is a tendency towards preference for individual finger involvement (lower indices of enslaving) accompanied by worse synergic control of fingers in multi-finger tasks. However, the control of individual finger force direction is impaired in the elderly as demonstrated by the increased index of finger force production orthogonal to the instructed direction. These findings may have implications for exercises to prevent the deterioration of the hand function with age and for rehabilitation of the hand function.