Theoretically, the mechanomyographic (MMG) mean power frequency fatigue threshold (MMG MPFFT) describes the maximal isometric torque that can be maintained for an extended period of time with no change in the global firing rate of the unfused, activated motor units. Purpose: The purposes of this study were twofold: (1) to determine if the mathematical model for estimating the electromyographic (EMG) MPFFT from the frequency of the EMG signal was applicable to the frequency domain of the MMG signal to estimate a new fatigue threshold called the MMG MPFFT; and (2) to compare the mean torque levels derived from the MMG MPFFT test for the vastus lateralis (VL), vastus medialis (VM), and rectus femoris (RF) muscles during isometric leg extension muscle actions. Methods: Nine adults (4 men and 5 women; mean±S.D. age=21.6±1.2 years) 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 exhaustion. Surface MMG signals were recorded from the VL, VM, and RF muscles during each fatiguing isometric muscle action. The MMG MPFFT was defined as the y-intercept of the isometric torque versus slope coefficient (MMG MPF versus time) plot. Results: There were no significant differences among the MMG MPFFT values for the VL, VM, and RF (34.8±23.4, 32.1±16.1, and 31.6±15.2Nm, respectively) muscles. Conclusion: The MMG MPFFT test may provide a non-invasive method to examine the effects of various interventions on the global motor unit firing rate during isometric muscle actions.
- Isometric leg extension
- MMG MPF fatigue threshold (MMG MPF)
- Mechanomyographic mean power frequency (MMG MPF)
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