The purpose of the present study was to examine the stationarity of surface mechanomyographic (MMG) signals from the vastus lateralis, rectus femoris and vastus medialis during concentric isokinetic, eccentric isokinetic and isometric muscle actions of the leg extensors. Eleven healthy men (mean +/- SD age = 20.1 +/- 1.1 years) performed submaximal to maximal concentric isokinetic, eccentric isokinetic and isometric muscle actions of the leg extensors, and surface MMG signals were detected from the vastus lateralis, rectus femoris and vastus medialis. All surface MMG signals were tested for stationarity with a procedure that examined changes in the mean value, variance, mean frequency and standard deviation of the power spectrum over time. The results showed that the three muscle action types resulted in similar levels of signal nonstationarity. Thus, despite the fact that the isometric muscle actions were performed at a constant force level and with no change in muscle length, the level of signal nonstationarity was similar to that for the concentric and eccentric isokinetic muscle actions. Future studies need to be done to identify the mechanism(s) causing MMG signal nonstationarity during isometric muscle actions.