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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 degrees 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.

To grasp with five digits of the hand requires an efficient parceling of contact forces in order to maintain static equilibrium as an object is lifted and held. In a previous study, subjects were asked to reach, grasp and lift a five-digit grip apparatus whose center of mass (CM) location was changed for each block of trials. Despite a modulation of force sharing patterns among the digits as a function of center of mass location, consistent in-phase and out-of-phase relationships between normal forces were found in the frequency domain. In the present study, we have used the same task to assess the effect of (a) predictability of an object's CM location (random vs blocked presentation) and (b) handedness (dominant vs non-dominant hand). Contrary to our original expectations, we found a similar modulation of normal forces to CM location during the hold phase across all conditions. Specifically, the force sharing pattern, i.e., the rank order of force contributed by each digit, emerged very early in the grasp sequence, remaining relatively stable throughout the duration of the lift and hold. Nevertheless, the extent to which force sharing patterns could be discriminated as a function of CM location was lower in the random than in the blocked conditions. Lastly, normal forces exerted by pairs of digits tended to be synchronized, both in-phase (thumb and fingers) and out-of-phase (pairs of digits) across a large proportion of the functional frequency range (up to 10 Hz) in all conditions. The composite of these findings suggests that the central nervous system uses stereotyped control strategies for coordinating multiple grip forces during grasping. Specific aspects of these schemes appear to be affected by predictability of object CM location, but not by hand dominance.

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