Two dimensional spatial coherence of skeletal muscle's natural vibrations during voluntary contractions.
Archer, Akibi A. A.
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Low frequency mechanical vibrations (<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). In this study, S-MMGs were recorded over a 3 x 5 grid of skin mounted accelerometers on the biceps brachii muscle during submaximal voluntary isometric contractions with the arm in a pronated position for ten healthy and young male subjects with no overt sign of neuromuscular diseases. For a given pair of accelerometers, the spatial coherence of S-MMG is a measure of the similarity of the S-MMG signals propagating between those two sensors. Two common techniques to estimate the spatial coherence for narrowband S-MMG signals, namely the magnitude squared coherence function and the maximum of the time-domain cross-correlation function, were found to yield similar results. In particular, high spatial coherence values were measured for sensor pairs aligned along the proximal to distal ends of the biceps, i.e. the longitudinal direction. On the other hand, the spatial coherence values for sensor pairs oriented perpendicular to the muscle fiber, i.e. along the transverse direction, were found to be significantly lower. This finding indicates that coherent S-MMGs were mainly propagating along the muscle fibers direction (longitudinal) of the biceps brachii within a frequency band varying between 10Hz to 50Hz. Additionally, the spatial coherence of S-MMGs along the longitudinal direction was found to decrease with increasing frequency and increasing sensor separation distance and to increase with contraction level varying between 20% to 60% of the maximum contraction level.