Relationship between mechanomyogram and force during voluntary contractions reinvestigated using spectral decomposition

A mechanomyogram (MMG) is considered to represent the pressure waves result ing from the lateral expansion of contracting muscle fibers. However, the a ctual MMG recording appears not only to reflect lateral changes of active f ibers, but also to include the effect of their longitudinal shortening, bec ause the fiber orientation, particularly in pennate muscles, is not paralle l with the MMG transducer attached at the skin surface. In the present inve stigation, a spectral decomposition method was developed to eliminate the i nterference due to fiber longitudinal movement from the MMG recording. The MMG was recorded over the belly of the rectus femoris muscle, which is a pe nnate muscle. Vibration over the tibial tuberosity (VTT) was used as a meas ure of the integrated longitudinal movement of the muscle fibers. The later al and longitudinal components included in the MMG were separated by a spec tral decomposition method that is based on the coherence function of the MM G and VTT. The MMG/force relationship was compared between the original and decomposed MMG. One-third of the 12 subjects demonstrated a curvilinear re lationship between the original MMG and force throughout the range of force . In the other two-thirds, the MMG saturated or reduced beyond 70% of the m aximum voluntary contraction (MVC). After decomposition, the MMG increased progressively with force up to 70% MVC, beyond which it decreased in all su bjects. The spectral decomposition method described here is considered to b e a useful tool with which to examine in more detail the MMG/force relation ship of different pennate muscles.