When the fingers need to act faster than the arm: coordination between grip force and load force during oscillation of a hand-held object.

Using a precision grip, 12 participants horizontally oscillated a lightweight object at different movement frequencies (1.0, 1.5, and 2.0 Hz) under two types of elastic load. In the first (CENT), the center of oscillation coincided with the neutral position of the object in the elastic force field, leading to two peaks in load force (LF) per cycle of movement (2:1 frequency ratio). In the second (BACK and FRONT), the neutral elastic force position of the object was located outside the range of movement, thus leading to only one LF peak per cycle of movement (1:1 frequency ratio). Results showed that in BACK and FRONT the coupling between grip force (GF) and LF (as reflected by coefficients of correlations) remained strong for all movement frequencies. In contrast, this coupling decreased in CENT as movement frequency increased, with participants switching progressively from two to one GF modulation per cycle of movement. Specific evaluation of performance under conditions giving rise to comparable LF frequencies (CENT at 1.0 Hz vs. BACK/FRONT at 2.0 Hz) confirmed the effect of frequency ratio on GF-LF coupling. We conclude that the control of GF is more efficient when LF varies at the frequency of movement than when it varies at twice this frequency, especially when movement frequency is high. These results are interpreted in the context of coordination dynamics and forward modeling approach.