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.