Longitudinal and transverse propagation of surface mechanomyographic waves generated by single motor unit activity

Multi-channel surface mechanomyographic (MMG) signals generated by individual motor units were analyzed to investigate whether the surface mechanical waves induced by fiber contraction propagate over the skin surface. The MMG signals were recorded from the tibialis anterior muscle of ten healthy subjects with 13 uniaxial accelerometers, located both along and transverse to the fiber direction. Intramuscular electromyographic signals served to identify individual motor units whose action potentials were used to trigger the averaging of the MMG signals. The spike-triggered averaged MMG had similar characteristics in locations along the longitudinal direction; however, its amplitude decreased along the transverse direction. Moreover, the time-to-positive peak increased along the transverse direction, indicating a transverse wave propagation with a velocity of 2.4 +/- 1.1 m/s in the linear direction. The results support the hypothesis that the MMG signal mainly originates from muscle fiber displacement underlining a bending mode due to contraction and provide the basis for interpreting the interference MMG in relation to motor unit activity.