Theoretically, the critical torque (CT) and electromyographic mean power frequency fatigue threshold (EMG MPFFT) demarcate fatiguing from non-fatiguing isometric torque levels. The purpose of this study was twofold: 1) to determine if the mathematical model for estimating the electromyographic fatigue threshold (EMGFT) from the amplitude of the EMG signal was applicable to the frequency domain of the EMG signal to estimate the EMG MPFFT; and 2) to compare the torque level derived from the CT test to that of the EMG MPFFT test for the vastus lateralis (VL) muscle during isometric muscle actions of the leg extensors. Nine adults (4 men and 5 women; mean ± SD age = 21.6 ± 1.2 yr) volunteered for this study. The first of five visits served as an orientation. Maximum voluntary isometric contraction (MVIC) was determined during the second visit. During visits 2-5, each subject performed one continuous, fatiguing isometric muscle action to exhaustion to determine the time to exhaustion (limit time; Tlim) at a randomly ordered percentage of MVIC (30%, 45%, 60%, or 75%). Surface EMG signals were recorded during each fatiguing isometric muscle action. The slope coefficient of the linear relationship between total isometric “work” (Wlim in N m s = Torque × Tlim) and Tlim was defined as the CT. The EMG MPFFT was defined as the y-intercept of the isometric torque versus slope coefficient (EMG MPF versus time) plot. The paired-samples t-tests indicated that there were no significant (p > 0.05) mean differences between absolute or %MVIC values for CT (25.3 ± 11.4 N·m and 17.6 ± 5.8 %MVIC) and EMG MPFFT (29.8 ± 22.9 N·m and 21.4 ± 8.7 %MVIC). The results of the present study indicated that there were no differences in the isometric torque levels associated with fatigue thresholds estimated from neuromuscular responses (EMG MPFFT) for the VL muscle and the torque versus duration relationship (CT). In addition, the current findings indicated that the mean CT occurred at a torque level (17.6 %MVIC) that is typically not affected by circulatory occlusion (20 %MVIC). It is likely, however, that continuous isometric muscle actions at the EMG MPFFT for the VL (21.4 %MVIC) muscle would be limited, in part, by restricted blood flow to the working muscles. These findings indicated that the mathematical model used to estimate the EMGFT during isometric muscle actions was applicable to the frequency domain of the EMG signal to estimate the EMG MPFFT. The tests (EMGFT and EMG MPFFT), however, utilize different domains of the EMG signal (the EMGFT test is based on fatigue-induced increases in EMG amplitude, while the EMG MPFFT test is based on decreases in EMG MPF). Therefore, the EMG MPFFT test can be used to examine neuromuscular fatigue characteristics associated with a decrease in muscle fiber action potential conduction velocity.