Mechanisms of skeletal muscle sounds: Acoustic measures of resonant frequency and tension.

Abstract

Contracting skeletal muscles emit pressure waves or sounds that are easily recorded with ordinary microphones. These sounds contain information regarding the contractile activity of muscle. The present study was designed to obtain a quantitative description of muscle sounds, determine the mechanism responsible for sound production, and develop a mathematical link between a muscle's characteristic sound and parameters of its contractile activity such as tension and stiffness. Four hypotheses were tested: (1) contracting skeletal muscle generates acoustic signals by vibrating laterally; (2) the vibrations occur at the muscle's resonant frequency; (3) changes in tension and elastic modulus determine changes in muscle resonant frequency; (4) wave equations predict the relationship among muscle resonant frequency, tension, and elastic modulus. Isolated preparations of frog skeletal muscle were used to test the hypotheses. Measurements of muscle tension, length, and sound were recorded simultaneously. Recordings of the pressure field produced by isometric contractions revealed that the sound pressure amplitude is proportional to the lateral acceleration of the muscle, inversely proportional to the square of the distance from the muscle, and cosinusoidally related to the major axis of lateral movement. The lateral vibration of the muscle occurs at its fundamental resonant frequency and has the appearance of an exponentially decaying chirp. Sound frequency spectrums were calculated using time-frequency transformations including the Wigner transform and Exponential distribution. Temporal mappings of muscle sound modal frequencies match the transverse resonant frequencies of muscles during twitch and tetanic contractions. During a tetanic muscle contraction the peak frequency initially increases and then becomes constant as the tension plateau is reached. The Rayleigh method was used to predict the relationship among measures of transverse muscle resonant frequency, elastic modulus and tension. Results show that elastic modulus for thin muscle has negligible influence on transverse muscle resonant frequency. The time course of muscle tension, but not elastic modulus, can be monitored acoustically during muscle contraction. Furthermore, muscle elastic modulus measured via length perturbations, is much larger than the elastic modulus that resists the lateral bending that causes sound production. The behavior of thin muscle is similar to a tensioned fibrous cable with distributed mass.