Does the fast Fourier transformation window length affect the slope of an electromyogram's median frequency plot during a fatiguing isometric contraction?

Reduced median frequencies (MDFs) are the hallmark of muscle fatigue in an electromyogram (EMG), the magnitude of which may be influenced by window lengths in fast Fourier transformation (FFT) algorithms used to compute power spectra. This study examined whether MDF reductions in fatigued muscles differed across varying FFT windows. EMG data were acquired from the gluteus maximus and semitendinosus muscles during a modified Biering-Sørensen test. Data were processed through varying FFT windows (0.1-, 0.5-, 1.0-, 2.0- and 5.0-s) and MDF slopes were compared with repeated measures analyses of variance. While FFT windows influenced variability in MDFs, the slopes did not differ across window lengths in either muscle group. When muscle fatigue is assessed via MDF slopes during submaximal isometric contractions, the FFT windows through which spectral characteristics are processed may have little bearing on results.

► Muscle fatigue is assessed via electromyogram (EMG) median frequency reductions. ► Fast Fourier transformation (FFT) algorithms are used to compute EMG power spectra. ► We examined how varying FFT window lengths affect median frequency reductions during fatiguing isometric contractions. ► Median frequency reductions did not differ across varying FFT window lengths (0.1-5.0 s) in either of two muscles tested. ► FFT window lengths may not affect the magnitude of EMG median frequency reductions in fatigued muscles.