Listening to muscle noise Available to Purchase

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...