Time-frequency analysis of mechanomyogram during sustained contractions with muscle fatigue

The mechanomyogram (MMG) is a recording of mechanical oscillation that is detectable on the body surface overlying the muscle. It is considered that the MMG is produced by lateral dimensional changes in active muscle fibers, which generate pressure waves, and reflects the mechanical activity of muscle. The present study was designed to examine the MMG behavior during sustained contraction at a constant level of force up to muscle fatigue, and to determine that the MMG reflects the specific motor unit (MU) activation strategy resulting from muscle fatigue. In the experiment, isometric contractions of biceps brachii muscles at 20 and 80% of the maximal voluntary contraction (20 and 80% MVC) were selected. The recorded MMG was analyzed by the short-time Fourier transform, and the root mean squared amplitude (RMS) and mean power frequency (MPF) were calculated every 0.1 s. It was found that the RMS of the MMG at 20% MVC decreased somewhat from the onset of contractions, then gradually increased. The MPF initially increased and then remained constant. At 80% MVC, on the other hand, the RMS continued to decrease. The MPF initially increased and then decreased. These trends in the RMS and MPF seem to reflect an alteration in the MU activation strategy resulting from muscle fatigue. The MMG is considered to be one of the useful tools in monitoring muscle fatigue. © 2003 Wiley Periodicals, Inc. Syst Comp Jpn, 35(1): 26–36, 2004; Published online in Wiley InterScience (). DOI 10.1002/scj.10528