Purpose: The purpose of this study was to determine the modulation of common low-frequency oscillations in pools of motor units across antagonistic muscles because of the difference in the activation level of pools of spinal motor neurons and the presence of neuromuscular fatigue during intended cocontraction.
Methods: Ten healthy young men (21.8 ± 1.5 yr) performed intended steady cocontractions of elbow flexors and extensors at maximal and a submaximal (10% of maximal EMG) effort. The submaximal cocontraction 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 EMG for the <3-Hz band and using rectified and unrectified EMG for the 3- to 15-Hz bands.
Results: The positive CCF peak in rectified EMG <3 Hz with little time lag (i.e., in-phase oscillations) during submaximal cocontraction was smaller compared with maximal cocontraction, but increased after the sustained contraction. In the 3- to 15-Hz band of both unrectified and rectified EMG, a negative CCF peak (i.e., out-of-phase oscillations) during submaximal cocontraction was smaller compared with maximal cocontraction 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- to 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 cocontraction, and 2) the magnitude of these correlated oscillations increases with the activation level of pools of spinal motor neurons and neuromuscular fatigue.