Mechanomyographic and electromyographic responses during fatiguing eccentric muscle actions of the leg extensors

The purpose of the current study was to examine the patterns of responses for torque, mechanomyographic (MMG) amplitude, MMG frequency, electromyographic (EMG) amplitude, and EMG frequency across 30 repeated maximal eccentric muscle actions of the leg extensors. Eleven moderately trained females performed an eccentric fatigue protocol at 30°/s with MMG and EMG signals recorded from the vastus lateralis. The results indicated there were significant (P < .05) decreases in MMG frequency (linear, r² = .395), EMG frequency (linear, r² = .177), and torque (linear, r² = .570; % decline = 9.8 ± 13.3%); increases in MMG amplitude (linear, r² = .783); and no change in EMG amplitude (r² = .003). These findings suggested that the neural strategies used to modulate torque during fatiguing eccentric muscle actions involved de-recruitment of motor units, reduced firing rates, and synchronization. In addition, the decreases in eccentric torque were more closely associated with changes in MMG frequency than EMG frequency. Thus, these findings indicated that MMG frequency, compared with EMG frequency, more accurately tracks fatigue during repeated maximal eccentric muscle actions.