Comparing electromyographic and mechanomyographic frequency-based fatigue thresholds to critical torque during isometric forearm flexion.

Theoretically, the critical torque (CT), electromyographic mean power frequency fatigue threshold (EMG MPF(FT)), and mechanomyographic mean power frequency fatigue threshold (MMG MPF(FT)) describe the maximal non-fatiguing isometric torque level.

Objective: The purposes of this study were threefold: (1) to determine if the mathematical model for estimating the EMG MPF(FT) during isometric muscle actions of the leg extensors was applicable to isometric muscle actions of the forearm flexors; (2) to determine if the mathematical model for estimating the EMG MPF(FT) 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 mechanomyographic mean power frequency fatigue threshold (MMG MPF(FT)); and (3) to compare the mean torque levels derived from the CT, EMG MPF(FT), and MMG MPF(FT) tests during isometric forearm flexion muscle actions.

Methods: Ten adults (4 men and 6 women, mean ± SD; age = 22.0 ± 2.1 years) performed three or four continuous, fatiguing, isometric muscle actions of the forearm flexors 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 Nms=torque × T(lim)) and T(lim) was defined as the CT. Surface EMG and MMG signals were recorded from the biceps brachii muscle during each fatiguing isometric muscle action. The EMG and MMG MPF(FT) were defined as the y-intercepts of the isometric torque versus slope coefficient (EMG and MMG MPF versus time) plots.

Results: There were no significant differences between fatigue thresholds (CT=26.3 ± 0.8, EMG MPF(FT)=31.4 ± 4.2, and MMG MPF(FT)=29.5 ± 7.0%MVIC), and the mean torque values (Nm) from the three fatigue thresholds were significantly intercorrelated at r = 0.94-0.96.

Conclusions: The EMG MPF(FT) test may provide a non-invasive method to examine the effects of interventions on the conduction velocity and shape of the action potential waveform. In addition, the effects of interventions on the global motor unit firing rate of the unfused, activated motor units may be examined by the non-invasive methods of the MMG MPF(FT) test.