The human central nervous system needs time to organize task-specific covariation of finger forces.

We studied how the central nervous system (CNS) organizes outputs of effectors in a redundant motor task. During four-finger ramp force production, finger forces show positive covariations across trials at low forces, which turn into negative covariations at a critical force value (F(CR)). Subjects performed such tasks with different target amplitudes and durations of the ramp. F(CR) showed significant linear relations to the rate of force change. The slopes of the relations varied across subjects corresponding to a critical time (T(CR)) ranging from 0.13 to 0.84 s. Across subjects, T(CR) showed no relation to maximal force production; T(CR) increased with the ramp duration. We conclude that the CNS needs a certain time to organize stabilization of total force by a negative covariation among finger forces.