Regulation of grasping forces during bimanual in-phase and anti-phase coordination.

When a hand-held object is moved, grip force is adapted in an anticipatory manner to load force due to a dynamic coupling between both forces. The present study addressed the issue of grip-load force regulation when moving rhythmically two hand-held objects in the vertical dimension, and more specifically the divergence of force control when performing according to the in-phase versus anti-phase mode. Results revealed that grip-load force ratio profiles were similar in both bimanual conditions. That is, force ratio was not constant throughout the movement cycles but followed a fairly regular pattern with maxima and minima, attained at upward and downward hand positions, respectively. However, anti-phase patterns showed an increased maximum grip-load force ratio as compared to in-phase patterns, whereas the latter did not differ from unimanual movements. The magnification of maximum force ratio during anti-phase movements suggests that rescaling occurred. This is likely due to the complexity of the anti-phase mode that necessitates increased monitoring and attention relative to the other performance conditions, creating a coordinative situation that imposes an additional degree of uncertainty. Therefore, the safety margin is amplified during anti-phase movements, probably as a strategy to prevent a potential destabilization of the grip during an asymmetrical load condition. Accordingly, these findings also demonstrate that grip-load force regulation is more proficiently controlled during bimanual in-phase than anti-phase movements. Herewith, the data add content to earlier work illustrating kinematic dissimilarities between both coordination modes.