Principal components of frequency domain electromyograms for muscular fatigue analysis

The parameters commonly employed for muscular fatigue (MF) analysis, the mean frequency (F(mean)) and median frequency (F(median)) of surface EMG (SEMG) spectra, did not present consistent results for exercises performed at low to moderate intensities. For overcoming this limitation, the present study proposes the use of principal component (PC) analysis of SEMG spectra for MF monitoring. The SEMG from vastus lateralis muscle of 24 young male subjects were recorded during a maximal effort test in cycle ergometer. Epochs of 200 ms SEMG, extracted from each period of muscle activation of each subject were used for estimating the amplitude spectra, which were employed to obtain the first two PCs. The PC coefficients from the first 40 spectra were used as a reference for calculating the standard distance. This index presented a consistent increase along the exercise, while the values of F(mean) and F(median) did not present any pattern. Comparisons between initial and final values from all subjects show no significant changes in F(mean) and F(median) (Student t test, P > 0.05) and a significant increase (P < 10(-3)) in standard distance, indicating this index as an alternative to MF analysis in cyclical exercises.