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

Although Hennemans size principle dictates an orderly small-to-large activation, evidence suggests a cer-tain level of flexibility in the recruitment of spinal motor neurons depending on task demands. Here, we investigate motor unit (MU) recruitment flexibility in the human first dorsal interosseous (FDI) muscle while controlling for overall muscle activation across two functionally distinct tasks using high-density in-tramuscular 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 (RTs) and recruitment orders (ROs) were highly consistent within the same task, but differed significantly between abduction and flexion. Across participants, 45.3% of MUs showed changes in recruit-ment above the coefficient of repeatability across tasks compared to only 5.0% within tasks. Changes in RT were accompanied by corresponding adaptations in discharge rate (DR), preserving the inverse RT-DR relationship. MU size did not have an effect on recruitment variability. 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 high-frequency inputs when FDI serves as a synergist muscle. Together, these results indicate different distributions of net excitatory input to FDI MUs across different functional tasks, that may include stronger involvement of spinal circuits dur-ing flexion as opposed to abduction, as indicated by the reduced intramuscular beta coherence. Moreover, the present findings also demonstrate that common synaptic input and not intrinsic motor neuron properties determines the inverse relation between MU RT and DR.