Correlated EMG Oscillations between Antagonists during Co-contraction in Men.

PURPOSE: The purpose of this study was to determine the modulation of common low-frequency oscillations in pools of motor units across antagonistic muscles due to the difference in the activation level of pools of spinal motor neurons and the presence of neuromuscular fatigue during intended co-contraction.

METHODS: Ten healthy young men (21.8 ± 1.5 yr) performed intended steady co-contractions of elbow flexors and extensors at maximal and a submaximal (10% of maximal electromyogram, EMG) effort. The submaximal co-contraction was repeated after sustained maximal contraction of elbow flexors. Surface EMG was recorded from the biceps brachii and triceps brachii muscles. Correlated EMG oscillations between the antagonistic muscles were quantified by the cross-correlation function (CCF) using rectified EMGs for the < 3 Hz band and using rectified and unrectified EMGs for 3-15 Hz bands.

RESULTS: The positive CCF peak in rectified EMG < 3 Hz with little time lag (i.e. in-phase oscillations) during submaximal co-contraction was smaller compared with maximal co-contraction, but increased after the sustained contraction. In the 3-15 Hz band of both unrectified and rectified EMGs, a negative CCF peak (i.e. out-of-phase oscillations) during submaximal co-contraction was smaller compared with maximal co-contraction, but increased after the sustained contraction. Across subjects, the degree of reduction in maximal EMG amplitude after the sustained contraction was correlated with the amount of change in the CCF peak in < 3 Hz, but not in the 3-15 Hz band.

CONCLUSION: The results indicate that 1) in-phase < 3 Hz and out-of-phase 3-15 Hz correlated EMG oscillations between antagonistic muscles occur during intended co-contraction, and 2) the magnitude of these correlated oscillations increases with the activation level of pools of spinal motor neurons and neuromuscular fatigue.