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 pre sent study addressed the issue of grip-load force regulation when moving rh ythmically two hand-held objects in the vertical dimension, and more specif ically the divergence of force control when performing according to the in- phase versus anti-phase mode. Results revealed that grip-load force ratio p rofiles were similar in both bimanual conditions. That is, force ratio was not constant throughout the movement cycles but followed a fairly regular p attern with maxima and minima, attained at upward and downward hand positio ns, 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 resealing occurred. This i s likely due to the complexity of the anti-phase mode that necessitates inc reased monitoring and attention relative to the other performance condition s, creating a coordinative situation that imposes an additional degree of u ncertainty. Therefore, the safety margin is amplified during anti-phase mov ements, probably as a strategy to prevent a potential destabilization of th e grip during an asymmetrical load condition. Accordingly, these findings a lso demonstrate that grip-load force regulation is more proficiently contro lled during bimanual in-phase than anti-phase movements. Herewith, the data add content to earlier work illustrating kinematic dissimilarities between both coordination modes. (C) 2001 Elsevier Science Ltd. All rights reserve d.