Differences in motor unit recruitment patterns and low frequency oscillation of discharge rates between unilateral and bilateral isometric muscle contractions.

Introduction: Distinct cortical activities contribute to unilateral and bilateral motor control. However, it remains largely unknown whether the behavior of motor neurons differs between unilateral and bilateral isometric force generation. Here, we first investigated motor units (MUs) recruitment patterns during unilateral and bilateral force generation. Considering that the force control is primarily regulated by low-frequency synaptic inputs to motor neurons, we also examined the relation between MU discharge rate and force output during unilateral and bilateral muscle contractions.

Methods: Using advanced electromyography (EMG) sensor arrays and spike-triggered averaging techniques, we examined a large population of MUs in the right first dorsal interosseous (FDI) muscle during unilateral and bilateral force tracking tasks. Using the principal component analysis, we analyzed the first common component (FCC) of MU discharge rate to describe the force fluctuations during unilateral and bilateral contractions.

Results: We found that MU discharge rate decreased during bilateral compared with unilateral contractions. MU recruitment threshold increased, while the amplitude and duration of MU action potential (MUAP) remained unchanged during bilateral compared with unilateral contractions. We found that the coefficients of variation (CV) for the force and FCC signal increased during bilateral compared with unilateral contractions. Notably, the FCC signal captured a great amount of MU discharge variability, and its CV correlated with the CV of the force signal.

Conclusion: Our findings suggest that MU recruitment patterns are altered during bilateral compared with unilateral isometric force generation, likely related to changes at the low-frequency portion of the synaptic drive.