When grasping and holding an object with five digits under a variety of task constraints, subjects use well-defined force coordination patterns, i.e., consistent force covariations and in-phase synchronization among all digit pairs. The question arises as to whether these force coordination patterns are default mechanisms for controlling multi-digit force production or whether they are specific to lifting and holding an object. To address this question, we asked subjects to grasp a manipulandum and exert forces with five digits simultaneously so as to match a force template measured from an actual object grasp, lift, and hold task (GLH). Unlike GLH, the force production task (FP) lacked the constraint of having to maintain object stability against gravity. The amplitude of individual finger forces and force covariations were similar for both tasks (with the exception of the little finger, which tended to produce less force in FP). Nonetheless, when multiple grip forces were not required to hold the manipulandum against gravity (FP), there was a significantly lower tendency for forces to be synchronized with higher intertrial variability of phase differences between forces exerted by all digit-pairs. Furthermore, the tendency for force phase differences to cluster at 0° was lower for FP than GLH. These results suggest that some aspects of the control of multi-digit grasping, i.e., force synchronization, are specific to object lift and hold rather than to the production of multi-digit forces. Modeling work suggests that motor unit synchronization might play an important role in the modulation of force synchronization patterns.
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