Wavelet-based intensity analysis of mechanomyographic signals during single-legged stance following fatigue

The von Tscharner (2000) “intensity analysis” describes the power of a non-stationary signal as a function of both frequency and time. The present study applied a version of this intensity analysis that utilizes Morlet wavelets as a means of gaining insight into the application of this technique as alternative to power spectral analysis for the evaluation of postural control strategy during the single-legged stance and to examine the effects of fatigue. Ten subjects (gender balanced, age: 25 ± 3 years; height: 169.4 ± 11.7 cm; weight: 79.0 ± 16.9 kg) participated in two trials consisting of five 15-s dominant-leg stances. Three-uniaxial accelerometers were fixed to the surface of the dominant leg corresponding to VM, VL, SOL, and MMG was recorded at a sampling rate of 1000 Hz. Signals were later analyzed using a variation of the von Tscharner intensity analysis consisting of a filter bank of 11 Morlet wavelets (range: 2.1–131.1 Hz). Two Wingate anaerobic tests (WAnT) separated by a 2-min rest were performed to introduce fatigue. Repeated measures ANOVAs showed significant effects for time, gender, trial, and wavelet (p < 0.001) and significant interactions for muscle by wavelet, gender by trial, trial by wavelet, and gender by trial by wavelet (p < 0.001). Peak total MMG intensity (mean ± SD) was higher in males than females and higher following fatiguing exercise preWAnT (squared m s−2): 42.6 ± 4.5 vs. 19.2 ± 2.3; postWAnT (squared m s−2): 90.4 ± 9.1 vs. 28.4 ± 2.8. Peak total MMG intensity was compressed to the lower frequencies surrounding ∼12 Hz, corresponding to what might be considered physiologic tremor, and a lower peak at ∼42 Hz was most prominent in SOL. The intensity analysis is a useful tool in exploring postural control and in studying the effects of fatigue on the mechanical properties of skeletal muscle.