Recording sound from human skeletal muscle: technical and physiological aspects.

In order to examine technical factors that influence muscle sound recording, single twitches of muscle were utilized since their waveforms were likely to be reproducible. We observed that satisfactory recordings could be made with either Archer air interface, or Hewlett-Packard direct contact sensor, microphones. Firm contact and stability between the microphone and the skin surface were particularly important. Frequencies below 20 Hz, the lower limit of the human auditory range, must be recorded, since they account for at least 90% of the power of the muscle sound wave. The chief frequencies were below 4 Hz. The sound wave produced by a maximal twitch of human thenar muscle induced by median nerve stimulation at the wrist is maximal in amplitude over the center and recedes to near zero at the margins of the muscle. It is preceded by the muscle compound action potential and is followed by the force curve, recorded with a strain gauge attached to the thumb. The sound resembles force in total time course, and it increases with increasing strengths of nerve stimulation. However, it differs in its latency, phase relationships, and response to nerve stimulation at different frequencies. Some of the features of muscle sound suggest it relates to both the active contractile and the parallel elastic components of muscle during a twitch contraction, but not the series elastic component.