“…This behavior of the myoelectric signal is called slow non-stationarity (or quasi-stationarity), given that the SEMG signal may preserve its characteristics during several seconds (Bonato et al, 2001), after which new muscle fi ber recruitment starts changing the signal properties (Farina, 2006). It is well established in scientifi c literature on fatigue analysis that under quasistationary conditions, the amplitude parameters (e.g., average rectifi ed value and root mean square) tend to increase, whereas frequency indicators (e.g., mean frequency and median frequency) tend to decrease during sustained fatiguing contraction (Lloyd, 1971;Moritani et al, 1982;Arendt-Nielsen and Mills, 1988;Kallenberg et al, 2007). However, under dynamic Volume 30, Número 4, p. 312-321, 2014 conditions the assumption of quasi-stationarity does not hold because the frequency content of the signal continuously changes over time.…”