abstract
- 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.