Control of finger force direction in the flexion-extension plane

We have examined the interaction among individual finger forces in tasks that required the production of the total force by a subset of fingers in a particular direction in the flexion-extension plane. Nine subjects produced fingertip forces in a prescribed direction with a maximum voluntary contraction (MVC) effort and held the peak force for two seconds. Six finger combinations were tested, four single-finger tasks--Index (I), Middle (M), Ring (R) and Little (L)--one two-digit task (IM), and one four-digit task (IMRL). The subjects were asked to generate the finger forces in two directions, 0 degrees (perpendicular to the surface of the transducer) and 15 degrees toward the palm. In all task conditions, there were two experimental sessions, with and without visual feedback on the task force vector. The main findings were: 1. The target direction significantly affected the constant error (CE) but not the variable error (VE) while removal of the feedback resulted in an increase in VE. 2. The direction of the forces produced by fingers that were not explicitly required to produce force (enslaved fingers) depended on the target direction. 3. In multi-finger tasks, the individual fingers produced force in directions that could differ significantly from the target direction, while the resultant force pointed in the target direction. There was a negative co-variation among the deviations of the directions of the individual finger forces from the target direction. If a finger force vector deviated from the target, another finger force vector was likely to deviate in the opposite direction. We conclude that a multi-finger synergy is involved in the control of the finger force direction.