A new EMG frequency-based fatigue threshold test

Theoretically, the critical torque (CT) and electromyographic mean power frequency fatigue threshold (EMG MPF_FT) describe the maximal non-fatiguing isometric torque level. Purpose: The purposes of this study were two-fold: (1) to determine if the mathematical model for estimating the EMG fatigue threshold (EMG_FT) from the amplitude of the EMG signal was applicable to the frequency domain of the EMG signal to estimate a new fatigue threshold called the EMG MPF_FT; and (2) to compare the torque level derived from the CT test to that of the EMG MPF_FT test for the vastus lateralis (VL) muscle during isometric muscle actions of the leg extensors. Methods: Nine adults (4 men and 5 women; mean ± SD age = 21.6 ± 1.2 yr) performed three or four continuous, fatiguing, isometric muscle actions of the leg extensors at 30, 45, 60, and 75% of maximum voluntary isometric contraction (MVIC) to determine the time to exhaustion (T_lim) values. The slope coefficient of the linear relationship between total isometric "work" (W_lim in N m s = Torque × T_lim) and T_lim was defined as the CT. Surface EMG signals were recorded from the vastus lateralis (VL) muscle during each fatiguing isometric muscle action. The EMG MPF_FT was defined as the y-intercept of the isometric torque versus slope coefficient (EMG MPF versus time) plot. Results: There were no significant differences between CT (19.7 ± 5.8%MVIC) and EMG MPF_FT (21.4 ± 8.7%MVIC). Conclusion: These findings provided indirect validation of the EMG MPF_FT test.