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 electromyogram (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: <i>r</i> = -0.78; EMG-based: <i>r</i> = -0.63, <i>P</i> < 0.025) but positively correlated with the fluctuations in the corresponding neural/electrical signal (MU-based: <i>r</i> = 0.54; EMG-based: <i>r</i> = 0.73, <i>P</i> < 0.05). In addition, the fluctuations in MU discharge frequency was negatively correlated with the fluctuations in net ankle extension-flexion force recorded as the mechanical output (<i>r</i> = -0.57, <i>P</i> < 0.05). Furthermore, correlated END strength was highly correlated between the MU-based and EMG-estimated measures (<i>r</i> = 0.83, <i>P</i> < 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.<b>NEW & NOTEWORTHY</b> Our results indicate that individuals with a greater correlated effective neural drive (END) to the antagonistic muscles exhibit smaller fluctuations in net force around the joint, suggesting steadier mechanical output during co-contraction. A high intra-correlation coefficient in the measure of correlated END between the use of surface EMG and MU discharge frequency during low-level co-contraction demonstrates a case in which the strength of correlated END can be observed in surface EMG in certain conditions.