MUSCLE SOUND FREQUENCIES OF THE FROG ARE MODULATED BY SKELETAL-MUSCLETENSION

The purpose of this study was to determine the relationships among mus cle sound frequencies, muscle tension, and stiffness. Time-frequency t ransformations of nonstationary acoustic signals provided measures of resonant frequency during isometric contractions of frog (Rana pipiens ) semitendinosus and gastrocnemius muscles. A mathematical expression for muscle transverse resonant frequency, elastic modulus and tension, based on elastic beam theory, was formulated by the Rayleigh method a dapted for muscles. For thin muscles, the elastic modulus was found to have negligible influence on transverse muscle resonant frequency. Ch anges in muscle tension were the major determinants of changes in tran sverse resonant frequency. Consequently, for thin muscles, the time co urse of muscle tension, but not elastic modulus, can be monitored acou stically during the early phase of contraction when muscles give rise to sounds. Muscles were found to be anisotropic with a modulus of elas ticity, E(L), measured via length perturbations near 0.1% muscle lengt h peak-to-peak, that was much larger than the modulus of elasticity, E (b), that resists the lateral bending that causes sound production. Th e elastic and resonant behavior of a thin muscle is similar to a tensi oned fibrous cable with distributed mass.