Do synergies decrease force variability? A study of single-finger and multi-finger force production.

We tested a hypothesis that force production by multi-finger groups leads to lower indices of force variability as compared to similar single-finger tasks. Three experiments were performed with quick force production, steady-state force production under visual feedback, and steady-state force production without visual feedback. In all experiments, a range of force levels was used computed as percentages of the maximal voluntary contraction force for each involved finger combination.

Feed-forward control of a redundant motor system.

We describe a model of feed-forward control of a redundant motor system and validate it using, as examples, tasks of multi-finger force production. The model assumes the existence of two input signals at an upper level of the control hierarchy, related and unrelated to a task variable. Knowledge of the Jacobian of the system is assumed at the level of generation of elemental variables (variables at the level of effectors).

Motor variability within a multi-effector system: experimental and analytical studies of multi-finger production of quick force pulses.

The purpose of the study was to develop a model of force variability for a fast action performed by a multi-effector system and to verify it for multi-finger quick force production. The experiments involved quick isometric contractions to different target force levels using different finger combinations. Force variance calculated over sets of trials for a multi-finger force production task showed non-monotonic single-peak profiles of force variance with a peak at a time between the times of the maxima of the force rate and of the total force.

Indices of nonlinearity in finger force interaction.

Experiments with force production by subsets of fingers within the human hand have shown that finger interaction may be significantly nonlinear. In particular, this nonlinearity is reflected in the phenomenon of force deficit, a drop of the peak force of a finger when several fingers act simultaneously. We describe nonlinear effects in force relations within finger pairs, triplets, etc.