Listening to muscle noise

Muscles make noise. For example, if you lean your ear on the palm of your hand, you can hear a low rumbling produced by the masseter muscle—a jaw muscle used for chewing. During voluntary contractions, such muscle noise, or physiological tremor, occurs naturally along the transverse direction of the muscle fibers due to the longitudinal shortening of the muscle’s actomyosin filaments. Karim Sabra and colleagues at the Scripps Institution of Oceanography have developed an elastography technique that uses measurements of the noise to determine muscle’s elastic properties. Standard elastography techniques estimate stiffness by using an active external source, such as indentation or an ultrasonic transducer, to generate propagating shear waves. The Scripps method, in contrast, is passive: Surface sensors, such as the skin-mounted accelerometers attached to the leg in the figure, record the intrinsic muscle noise. Cross-correlating the signals from those accelerometers clearly revealed a propagating shear wave traveling down the leg; from the phase velocity, the experimenters could extract muscle properties—the absolute shear modulus (that is, stiffness) and the dynamic shear viscosity—as the muscle contracted. This low-cost, noninvasive technique for measuring muscle stiffness, say the researchers, can be used to diagnose and monitor muscle activity and can provide information about muscular dystrophy and other neuromuscular diseases. (K. G. Sabra et al., Appl. Phys. Lett. 90 , 194101, 2007 .)