Muscles make noise. For example, you can hear the sound of the masseter muscle-a jaw muscle used in chewing food-by propping your head (ear down) in the palm of your hand. The low rumbling comes from the shortening of the actomyosin filaments in the muscle fibers. Muscle noise can be measured using various sensors, such as microphones and even skin-mounted accelerometers.
Scientists at the Scripps Institute of Oceanography listen to muscle noise in order to detect muscle stiffness, which in turn can provide information about neuromuscular disease, such as muscle dystrophy. Muscle stiffness was traditionally measured using external radiation sources (such as a vibrating piston).
But the Scripps researchers use a process called passive elastography, a low-cost, in-vivo, non-invasive technique in which an array of surface sensors follow the passing of natural shear waves traveling along the muscle fibers. The new results will be delivered next week by Karim Sabra (firstname.lastname@example.org) at the meeting of the Acoustical Society of America (ASA) taking place June 4-8 in Salt Lake City.
By the way, the Scripps scientists were originally interested in underwater noise effects and only later adapted their work to noise in muscle. (ASA paper 2pUW9; meeting website at http://www.acoustics.org/press)