Abstract
PURPOSE: The physical work capacity at the fatigue threshold (PWCFT) is determined from fatigue-induced increases in the amplitude of the surface electromyographic (EMG) signal, while the mean power frequency fatigue threshold (MPFFT) is estimated from the compression of the power density spectrum of the signal. The purpose of this study was to compare the mean power outputs from the PWCFT and MPFFT during incremental cycle ergometry. METHODS: Ten subjects (mean ± SD age = 23.3 ± 3.7 yrs) performed an incremental cycle ergometer test to exhaustion with bipolar surface EMG signals recorded from the vastus lateralis muscle. During each 2 min power output of the incremental test, six, 10 s EMG samples were recorded. The EMG amplitude and EMG MPF values were calculated for each of the 10 s epochs and separately plotted across time for each power output of the test. The PWCFT was determined by averaging the highest power output that resulted in a non-significant (p > 0.05; single-tailed t-test) slope coefficient for the EMG amplitude versus time relationship and the lowest power output that resulted in a significant (p < 0.05) positive slope coefficient. The MPFFT was determined by averaging the highest power output that resulted in a non-significant (p > 0.05; single-tailed t-test) slope coefficient for the EMG MPF versus time relationship and the lowest power output that resulted in a significant (p < 0.05) negative slope coefficient. RESULTS: The results of a paired t-test indicated that the mean power output for the MPFFT (177.5 ± 64.8 W) was significantly greater (p < 0.05) than that of the PWCFT (152.5 ± 48.9 W). CONCLUSIONS: The significantly greater mean power output for the fatigue threshold based on the frequency (MPFFT) of the surface EMG signal than for the amplitude (PWCFT) suggested that the MPFFT and PWCFT reflect different dimensions of neuromuscular fatigue. It is possible that the PWCFT demarcates the moderate from heavy exercise intensity domains, while the MPFFT may demarcate the heavy from severe intensity domains.
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