Beta-band Motor Unit Coherence and Nonlinear Surface EMG Features of the First Dorsal Interosseous Muscle Vary with Force

Motor unit (MU) firing times are weakly coupled across a range of frequencies during voluntary contractions. Coherent activity within the beta-band (15-35 Hz) has been linked to oscillatory cortical processes, providing evidence of functional connectivity between the motoneuron pool and motor cortex. The aim of this study was to investigate whether beta-band MU coherence is altered with increasing abduction force in the first dorsal interosseous muscle. Coherence between MU firing times, extracted from decomposed surface EMG signals, was investigated in 17 subjects at 10%, 20%, 30% and 40% of maximum voluntary contraction. Corresponding changes in nonlinear surface EMG features, specifically sample entropy and determinism which are sensitive to MU synchronization, were also examined. A reduction in beta-band and alpha-band coherence was observed as force increased (F(3, 151) = 32, p < .001 and F(3, 151) = 27, p < .001, respectively), accompanied by corresponding changes in nonlinear surface EMG features. A significant relationship between the nonlinear features and MU coherence was also detected (r = -0.43 ± 0.1 and r = 0.45 ± 0.1, for sample entropy and determinism, respectively, both p < .001). The reduction in beta-band coherence suggests a change in the relative contribution of correlated and uncorrelated pre-synaptic inputs to the motoneuron pool, and/or a decrease in the responsiveness of the motoneuron pool to synchronous inputs at higher forces. The study highlights the importance of considering muscle activation when investigating changes in MU coherence or nonlinear EMG features, and examines other factors that can influence coherence estimation.