Two dimensional characterization of skeletal muscle’s natural vibrations during voluntary contractions.

Abstract

During voluntary muscular contractions, low frequency mechanical vibrations (<100 Hz) are naturally generated by skeletal muscles as a result of the muscle fiber activity and the overall change of the muscle-tendon geometry. Determining the temporal and spatial variations of muscle vibrations could reveal fundamental features of skeletal muscles’ physiology and activation mechanisms. These mechanical vibrations were recorded over a 3 × 5 grid of skin mounted accelerometers on the biceps brachii muscle for ten healthy subjects for various contraction levels during isometric elbow flexion. The spatial origin and propagation directionality of these natural muscle vibrations were characterized across frequencies using standard cross correlation techniques and back projection algorithms (akin to time-reversal imaging). Additionally cross-correlating these accelerometer recordings provided travel-time measurements of these natural muscle vibrations between multiple sensor pairs. Furthermore, travel-time tomographic inversions yielded spatial variations of their propagation velocity from which the local stiffness of the muscle could be estimated using elastography principles. This passive tomographic elastography technique could potentially lead to simpler, low-cost, and noninvasive monitoring of musculoskeletal and neuromuscular disorders, without using an external mechanical or radiation excitation as commonly required by conventional elastography techniques.