SUMMARY
Low frequency mechanical oscillations (<100 Hz) are naturally generated by skeletal muscles during voluntary contractions. Recording of these vibrations at the muscle surface are called surface mechanomyograms (S-MMGs). This study analyzes the two-dimensional spatial coherence and propagation directionality of the S-MMGs recorded over a 3 x 5 grid of skin mounted accelerometers on the biceps brachii muscle during sub-maximal voluntary isometric contractions. The directionality of the propagating S-MMGs was found to be independent of frequency. 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. On the other hand, measuring spatial coherence values for sensor pairs perpendicular to the muscle fibers, did not exhibit result in high spatial coherence values, meaning that there was not a specific directionality. A simple model describing the variations across range of the S-MMG spatial coherence was subsequently used to invert for the frequency variations of the average phase velocity of the propagating S-MMG along the biceps brachii axis. These results indicated that the spatial coherence of S-MMG propagating along skeletal muscles could provide a quantitative mean for passive sensing and imaging of the visco-elastic properties of skeletal muscles.