Comparison of a New Mechanomyographic Frequency-Based Fatigue Threshold Test and Critical Torque

Abstract

Theoretically, the critical torque (CT) test estimates the maximal, non-fatiguing isometric torque level. In addition, it has been suggested that changes in the mechanomyographic (MMG) mean power frequency (MPF) provide qualitative information regarding the global firing rate of the activated motor units. PURPOSE: The purposes of this study were twofold: 1) to determine if the mathematical model for estimating the electromyographic (EMG) fatigue threshold (EMGFT) from the amplitude 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 torque level derived from the CT test to that of the MMG MPFFT 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 (Tlim) values. 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. Surface MMG signals were recorded from the VL 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 (p 0.05) between absolute (25.2 ± 11.4 and 34.8 ± 23.4 N·m, respectively) and %MVIC (19.7 ± 5.8 and 23.7 ± 7.8 %, respectively) values for CT and MMG MPFFT. CONCLUSION: The non-significant mean difference between CT and MMG MPFFT suggests that continuous isometric leg extension muscle actions at the MMG MPFFT could be maintained for an extended period of time without fatigue. Furthermore, 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.