Two strategies of anticipatory mechanism during change in frequency of cyclic multi-finger force production in isometric conditions

Cyclic force production by multiple fingers requires proper organization of finger forces, i.e., synergy, while concurrently accommodating predictable or anticipatory changes in movement frequency. A plethora of studies have been conducted on this subject, employing an experimental paradigm of the quick pulse force production subsequent to the steady-state condition, while the underlying mechanism of reorganization of multi-finger synergies remains unclear in response to required cyclic dynamics changes. In the present study, the time-varying multi-finger synergy and its anticipatory adjustment were investigated in healthy young adults who generated cyclic four-finger flexion at 0.5 and 2 Hz under "frequency-maintained" and "frequency-changed" conditions. The participants were informed of the transition in advance. Synergy indices in time-series were quantified across multiple trials using the uncontrolled manifold framework. The synergy indices demonstrated transient decreases exclusively during the "frequency-changed" frequency conditions, indicating a temporary weakening of force-stabilizing coordination around the frequency transition. The decrease manifested earlier and more distinctly when frequency changed from high-to-low, whereas in the low-to-high transition, it was delayed by about 30 ms. The observed synergy drop was driven primarily by increased variance orthogonal to the uncontrolled manifold, with relatively little change in variance within the uncontrolled manifold. These findings suggest that predictable cyclic force transitions are accommodated through temporally specific reorganization of multi-finger coordination, expressed mainly as a transient increase in task-relevant variance.