Adaptations in common synaptic inputs to spinal motor neurons during grasping versus a less functional hand task

Previous evidence suggests that shared synaptic inputs across spinal motor neurons play a key role in coordinating multiple muscles during hand movements, reducing control complexity. In this study, we investigated how the nervous system modulates these common synaptic inputs during a functionally relevant grip (grasping) compared with less functionally relevant hand tasks. Seventeen participants performed three different tasks: simultaneous four-finger flexion without thumb involvement (four-finger flexion), thumb flexion, and simultaneous flexion of both fingers and thumb (grasping). For each task, subjects sustained isometric contractions at 5% and 15% of maximal voluntary contraction, whereas high-density surface electromyograms (HDsEMG) were recorded from the extrinsic flexor muscles of the hand. Motor unit spike trains were decomposed from HDsEMG and tracked across tasks, and their mean discharge rate was calculated. Coherence between motor units was quantified within the delta, alpha, and beta bands to estimate common synaptic oscillations. At both force levels, the mean discharge rate decreased during grasping compared with four-finger flexion but increased during grasping compared with thumb flexion. In addition, the area under the curve of coherence within the alpha band decreased by ~20% during grasping compared with the four-finger flexion task, with no significant delta or beta bands changes. These reductions in alpha band coherence were reflected in force oscillations, showing decreased force-neural drive coupling within the alpha band and increased force steadiness during grasping compared with four-finger flexion. Our findings suggest that a functionally relevant and frequently used grip involves distinct neural control mechanisms that ultimately enhance force control. NEW & NOTEWORTHY Our results demonstrate that grasping, which is a functionally relevant and habitually performed hand movement, exhibits reduced alpha band oscillations in common synaptic inputs compared with less functionally relevant grips. Importantly, these differences were reflected in force oscillations, revealing decreased force-neural drive coupling within the alpha band and increased force steadiness during grasping. These findings suggest that a more natural grip involves specific neural control mechanisms to enhance force control.