Initial mechanomyographical signals from twitching fibres of human skeletal muscle

To elucidate the origin of characteristic low-frequency sound of contracting skeletal muscle, propagation properties of mechanical and electrical signals along twitching muscle were studied.

From voluntarily twitching muscle in human vastus medialis muscle, mechanical and electrical signals were simultaneously recorded through the skin with microphones and wire electrodes.

The initial part of the mechanical signal recorded by the microphone showed transient pressure decrease (negative-going phase) followed by sustained pressure increase (positive-going phase). The mechanical signals obtained over the nerve insertion lacked the negative-going phase. The onset of the positive-going phase appeared 2.5 +/- 0.2 (mean +/- SEM, n = 7) ms behind the onset of spike potential independently of the sensor position. The onset of the negative-going phase propagated along the muscle at a constant velocity of 29.8 +/- 3.4 m s(-1), which was about ten times faster than the propagation velocity of action potentials (3.2 +/- 0.1 m s(-1)). Extrapolation analyses showed that the negative- and positive-going phases originated at the nerve insertion 2.1 +/- 0.3 and 1.6 +/- 0.1 ms behind the generation of muscle action potentials respectively. Local muscle compression blocked the propagation of only the negative-going phase.

The negative-going mechanical signal represents a passive process, probably the thinning of the internally stretched segments of the twitching muscle. The positive-going phase, on the other hand, represents an active process, probably muscle thickening due to the internal shortening. Each phase, therefore, would convey information about the mechanical properties of resting and contracting muscle fibres respectively.