Influence of Force Direction on Motor-Unit Activity During Low-force Isometric Contractions with the First Dorsal Interosseous Muscle

The purpose of this study was to compare the discharge characteristics of motor units in the first dorsal interosseous (FDI) muscle when it performed low-force isometric contractions in the directions of index finger abduction, flexion, or concurrent abduction + flexion (combined). Motor-unit activity was identified from high-density surface electromyography recordings obtained with grid electrodes attached to the skin over FDI while participants performed low-force isometric contractions at 10% of maximum in the three directions. A total of 378 motor units were identified across all participants and tasks with an average of 10.4 motor units per participant tracked across 2–3 tasks. Mean and peak discharge rates were greater during abduction than during flexion or the combined task, whereas discharge rates at recruitment and derecruitment were lower during abduction. Factor analysis identified a single dominant mode that explained >93% of the variance in motor-unit discharge rates across all directions despite directional differences in Mode 1 loadings, discharge characteristics, and correlations between motor-unit-derived signals and fluctuations in force. These findings indicate that the motor-unit discharge rates in FDI during low-force isometric contractions are organized within a single dominant low-dimensional structure that is preserved across force directions despite the modulation of discharge characteristics across directions.