Two dimensional spatial coherence of the natural vibrations of the biceps brachii muscle generated during voluntary contractions

Surface mechanomyograms (S-MMGs) are recorded from low frequency (< 100 Hz) mechanical oscillations that are naturally generated by skeletal muscle during voluntary contractions. This study investigates a method to determine the propagation directionality of the S-MMG waves. A 3x5 grid of skin mounted accelerometers was mounted on the biceps brachii muscle during submaximal voluntary contractions. This method resulted in findings that the propagation directionality of the S-MMGs are frequency dependent. At high frequencies (> 25 Hz), high spatial coherence values were only measured for sensor pairs aligned along the proximal to distal (i.e. longitudinal) orientation, thus indicating that coherent S-MMG were mainly propagating along the muscle fibers direction of the biceps brachii at those frequencies. On the other hand, at lower frequencies (< 25 Hz), the S-MMG spatial coherence values did not exhibit a specific directionality. This method provides results that have an important implication of finding the average phase velocity of the propagating S-MMG wave, which can be used to determine viscoelastic properties of skeletal muscles.