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

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.