Association between correlated effective neural drive and electromechanical fluctuations during co-contraction: Insights from MU discharges and interference EMG.

Abstract

This study investigated the correlation between the strength of correlated effective neural drive (END) to the antagonistic muscles and the fluctuations in neural/electrical and mechanical output around the joint during steady co-contraction, and whether the correlated END strength estimated from conventional surface EMG is correlated with that determined from motor unit (MU) discharges. Fourteen young male participants performed isometric steady co-contractions with their medial gastrocnemius and tibialis anterior muscles at 10% of maximal EMG while sitting. Correlated END strength was quantified as the maximum value of the cross-correlation function between the conventional surface EMG signals and between MU discharges decomposed from high-density surface EMG of each muscle. Correlated END strength quantified from each signal was both negatively correlated with the fluctuations in net force (MU-based: r = -0.78; EMG-based: r = -0.63, P < 0.025) but positively correlated with the fluctuations in the corresponding neural/electrical signal (MU-based: r = 0.54; EMG-based: r = 0.73, P < 0.05). Additionally, the fluctuations in MU discharge frequency was negatively correlated with the fluctuations in net ankle extension-flexion force recorded as the mechanical output (r = -0.57, P < 0.05). Furthermore, correlated END strength was highly correlated between the MU-based and EMG-estimated measures (r = 0.83, P < 0.05). These results suggest that individuals with a greater correlated END to the antagonistic muscles exhibit greater neural/electrical fluctuations yet steadier mechanical output at the joint level during co-contraction, and the strength of correlated END can be observed in conventional surface EMG in such conditions.