The control of whole hand grasping relies on complex coordination of multiple forces. While many studies have characterized the coordination of finger forces and torques, the control of hand muscle activity underlying multi-digit grasping has not been studied to the same extent. Motor-unit synchrony across finger muscles or muscle compartments might be one of the factors underlying the limited individuation of finger forces. Such "unwanted" coupling among finger forces, however, might be desirable when a high level of force coupling is required to prevent object slip during grasping. The goal of this study was to quantify the strength of synchrony between single motor units from extrinsic hand muscles as subjects held a device with a five-digit grasp. During the hold phase, we recorded the normal force exerted by each digit and the electrical activity of single motor units from each of the four divisions of the muscle flexor digitorum profundus (FDP) and one thumb flexor muscle, m. flexor pollicis longus (FPL). The strength of motor-unit synchrony was quantified by the common input strength index (CIS). We found moderate to strong motor-unit synchrony between FPL and the index FDP compartment [CIS: 0.49 ± 0.03 (SE)] and across most FDP compartments (0.34 ± 0.02). Weak synchrony, however, was found between FPL and the middle, ring, and little finger FDP compartments (0.25 ± 0.01). This difference might reflect the larger force contribution of the thumb-index finger pair relative to other thumb-finger combinations in five-digit grasping.
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