Strength training increases training-specific multi-finger coordination in humans

Jae Kun Shim, Jeffrey Hsu1, Sohit Karol, Ben F. Hurley

pending publication as of 10/16/09

ABSTRACT
The purpose of the present study was to investigate the effects of finger Strength Training (ST) on finger strength, independence, force control and adaptations in multi-finger coordination. Thirty-three healthy, young (23.0 ± 2.9 yrs) subjects were randomly assigned into four groups. Group 1 (G1) trained all fingers together, group 2 (G2) trained individual fingers without restricting movements of the non-training fingers, and group 3(G3) trained individual fingers while restricting the movement of the non-training fingers.The control group (G0) did not undergo any training. A vertically hanging load was attached to a spring which passing through a pulley. The other end of the string extended to the horizontal plane, and had thimbles attached to it. Subjects were asked to rest their forearm on the table and lift the load by inserting their fingers into the thimbles. The training protocol lasted six-weeks. Identical experimental tests were conducted four times,biweekly, across the six-week training. Force coordination and moment coordination,defined as synergies stabilizing the resultant force and the resultant moment of all finger forces, in a multi finger pressing task were quantified using the Uncontrolled Manifold (UCM) analysis. The UCM analysis allocates motor variability into two components, one in the null space of a motor task and the other perpendicular to the null space. During multi-finger pressing tasks, multi-finger coordination exists when the variability in the null space is greater than the variability in the subspace perpendicular to the null space.The multi-finger coordination was quantified as the difference between the variance within the null space and that perpendicular to the null space , normalized by the total variance. Thus, the coordination measure in our analysis is a unit less variable. A greater coordination measure indicates better multi-finger coordination. Moment stabilizing multi-finger coordination increased only in G1 (from 1.197 ± 0.004 to 1.323 ± 0.002, P <0.01), while force stabilizing coordination increased only in G3 (from 0.207 ± 0.106 to0.727 ± 0.071, P < 0.01). Finger strength, measured by the maximal voluntary finger force of pressing four fingers, increased significantly in all training groups (from 103.7 ±3.1 N to 144.0 ± 3.6 N for training groups, all P < 0.001). Finger force errors, quantified by the deviations between the required force profiles (20% MVF) presented to the subjects and the actual force produced, decreased significantly with ST for all the training3groups (all P < 0.05). Finger independence also decreased significantly for all the training groups (P < 0.05). We conclude that the neuromuscular system adaptations to multifinger ST are specific to the training protocol being employed, yielding improvements indifferent types of multi-finger coordination (i.e., coordination-specific ST), finger force control, finger strength and a decrease in finger independence. Finger independence,depending on the nature of the task, may or may not be favorable to certain task performances. We suggest that ST protocol should be carefully designed for the improvement of specific coordination of multi-effector motor systems.