RESUMO
The present study was assigned to compare the applicability and sensitivity of three different methods (discrete wavelet transforms (DWT), median frequency (MDF) - calculated on the basis of fast Fourier transform, and spectral indices) for analysis of frequency content changes in potentials produced by repetitive stimulation from isolated slow (SMFs)- and fast (FMFs)-fatigable muscle fibers during uninterrupted activity. In order to affect fatigue, prior to starting the fatiguing stimulation (5 Hz), some of the fibres were exposed to microwave electromagnetic field and others were sham exposed. All the methods studied demonstrated that during a long-lasting fibre activity, the shift of the potential frequency content to lower frequencies was a consequence of lowering in higher frequency components and increasing in low frequency ones. The changes were faster in FMFs than SMFs and stronger in sham exposed fibres vs. microwave exposed. The MDF alterations in both fibre types and protocols were quite similar to other methods characterizing either individual changes in low and high frequency scales of the signal (discrete wavelet transform - DWT) or changes in their spectral ratio (spectral indices). The spectral indices showed sensitivity to fatigue effect at the final stages that made their changes considerably non-linear. DWT allowed processing the temporal occurrence of frequency content changes that corresponded to the alterations in the separate potential phases as well as earlier detection and precise evaluation of fatigue onset and progressing. All methods are applicable but DWT is a preferable for study of muscle fibre fatigue in clinical and experimental neurophysiology.
