The emergence and disappearance of multi-digit synergies during force-production tasks

We analyzed patterns of covariation among forces produced by the five digits of the human hand during tasks that required the production of a pattern of the total force consisting of ramp-up, constant force, and ramp-down segments with the time of the ramps ranging from 0 to 3000 ms. Patterns of the variance of the total force and the sum of the variances of individual digit forces were compared over sets of 12 trials at each task. The initiation of the ramp-up segment was associated with positive covariation of digit forces. Negative covariation among digit forces (force-stabilizing synergies) emerged after a critical time of 600-800 ms, which was only weakly dependent on the ramp time. These synergies persisted over the steady-state phase. A quantitative index of digit force covariation was introduced; it showed a drop about 100 ms before initiation of the ramp-down phase; we termed this phenomenon "anticipatory covariation" (ACV). The ramp-down phase was associated with rapid disappearance of the force-stabilizing synergy over a time period that ranged from 0 to 600 ms and scaled strongly with the duration of the force ramp. Thumb-virtual finger synergies showed qualitatively similar behavior to the multi-finger synergies (virtual finger is an imagined digit whose action is mechanically equivalent to the action of the four fingers). We conclude that abrupt changes in a time profile of total force are associated with transient destabilization of the total force. Changes in force-stabilizing synergies may occur in preparation to changes in the total force.