Muscular sound is a mechanical phenomenon detectable at the surface of an active muscle, which has been known and described since 1800. Only recently, because of the availability of reliable transducers and sophisticated analysis techniques, has this signal become attractive for monitoring the mechanical aspects of muscle contraction. The muscular sound characteristics were investigated both during electrically elicited and voluntary contractions. In the first case, the influence of the biophysical and mechanical properties of the muscle on this signal was studied. During voluntary efforts the summation of the mechanical activity of the recruited motor units was analyzed. The results indicate that the muscular sound may be an adjunct tool to the force, the physiological force tremor, and electromyogram to obtain information on the muscle mechanical model as well as on muscle motor control. This review focuses on the following aspects of the signal: (1) recording problems; (2) muscle sound properties during stimulation of isolated and in vivo muscle; (3) time and frequency domain analysis during non fatiguing and fatiguing contractions; (4) comparison with other signals related to muscle activity; (5) discussion on the origin; and (6) possible practical applications.
