Task-Dependent Motor Unit Recruitment and Rate Coding Reveal Redistribution of Neural Drive in the Human Hand

Although Henneman's size principle dictates an orderly small-to-large activation for a given common input, evidence suggests a certain level of flexibility in the recruitment of spinal motoneurons depending on task demands. Here, we investigate motor unit (MU) recruitment flexibility in the human first dorsal interosseus (FDI) muscle while controlling for overall muscle activation across two functionally distinct tasks using High-Density intramuscular EMG (HD-iEMG) electrode arrays. Six participants performed isometric index finger abduction where the FDI serves as the prime mover, and flexion with the FDI functioning as a synergist. Recruitment thresholds (RT) and recruitment orders (RO) were highly consistent within the same task, but differed significantly between abduction and flexion. Across participants, 45.3% of MUs showed changes in recruitment above the coefficient of repeatability across tasks compared to only 5.0% within tasks. The spatial distribution of muscle activation did not shift between tasks, confirming that the same muscle region was engaged in both. Changes in RT were accompanied by corresponding adaptations in discharge rate (DR), preserving the inverse RT-DR relationship. Intramuscular coherence analysis revealed no differences in the delta (1-5 Hz) or alpha (5-13 Hz) band, but beta band (13-30 Hz) coherence was significantly lower during flexion than abduction, indicating reduced common input synchronization in this frequency range when FDI serves as a synergist muscle. Together, these results indicate different distributions of net excitatory input to FDI MUs across different functional tasks, potentially involving a stronger contribution of spinal circuits during flexion, as suggested by the reduced intramuscular beta coherence. Moreover, the present findings also demonstrate that common synaptic input and not intrinsic motoneuron properties determines the inverse relation between MU RT and DR.