Force variability and neural control differences in an upper and lower limb muscle.

The purpose of this study was to compare force variability and motor unit firing behavior between the first dorsal interosseous (FDI) and tibialis anterior (TA) muscle, and between sexes. Twelve healthy males (age: 22.7 ± 2.7 yr, height: 1.8 ± 0.1 m; weight: 70.5 ± 18.5 kg) and 12 healthy females (age: 21.4 ± 1.9 yr; height: 1.6 ± 0.04 m; weight: 64.6 ± 10.6 kg) participated in this study. Participants completed a series of force tracing tasks, including steady force and varying force, by abducting their index finger and dorsiflexing their foot at submaximal intensities while force and motor unit behavior were recorded. Muscle-related differences in the coefficient of variance (CV) of force (P ≤ 0.02) were sex- and task-specific. The coefficient of variation of motor unit interspike intervals (CVISI) was higher in the FDI than the TA during both the constant force and force-varying contractions (P < 0.01). The CVISI was greater in males than females during both tasks, in the TA only (P < 0.01). Neural control may differ between muscles and sexes, in a task-dependent manner.NEW & NOTEWORTHY This study provides a unique, comprehensive comparison of the control of force and motor unit firing rates between a muscle in the upper and lower limb, and between sexes. Our findings identify differences in neural control between muscles and sexes; however, these differences were not observed in force control.