Muscle sounds are emitted at the resonant frequencies of skeletal muscle

The changes in mechanical resonant frequency of whole muscles during twitch and tetanic contractions are compared to changes in frequency components of the pressure wave produced by muscles during contraction. Resonant frequencies were determined by imposing sinusoidal length changes on a muscle and observing transverse standing waves when the frequency of length change matched the muscle´s resonant frequency or a harmonic of the resonant frequency. Acoustic signal instantaneous frequency spectra were calculated using time-frequency transformations including the Wigner transform and the exponential distribution. During a tetanic muscle contraction, the peak instantaneous frequency initially increased and then became constant as the force plateau was reached. The resonant frequencies determined during the force plateau and during relaxation spanned the same range as the peak instantaneous frequency of the acoustic signal. These results suggest that the acoustic signal may be useful as a noninvasive monitor of muscle resonant frequency during contraction.