TIME-FREQUENCY ANALYSIS OF SKELETAL-MUSCLE AND CARDIAC VIBRATIONS

Skeletal muscle and the heart vibrate during contraction producing non stationary signals whose time-varying frequency reflects dynamic chang es in physiological properties. Consequently, pathological changes in the mechanical integrity or lending of skeletal muscle or the heart ca n be expected to alter their vibrations.' Classic frequency analysis t echniques have been inadequate to characterize these subtle changes be cause of rapidly varying frequency components. A poor understanding of heart and muscle sound generation has also limited investigations. Th is paper demonstrates how time-frequency (TF) techniques have illumina ted the relationships between muscle/heart material properties and loa ding, and frequency dynamics of heart and muscle vibrations. Studies o f evoked twitches from frog skeletal muscle reveal that muscle vibrati ons occur as transverse oscillations at the muscle's resonant frequenc y. Using a classic Rayleigh-Ritz model and crude estimates of the musc le geometry muscle force can be accurately predicted from the muscle s ound TF profile. First heart sound vibrations, in contrast, are shown to be a nonresonant phenomena consisting of propagating transients sup erimposed upon bulk acceleration of myocardial contraction. Consequent ly, first heart sound frequency dynamics depend upon cardiac electrica l excitation and hemodynamic loading in addition to intrinsic material properties and geometry, necessitating further work to characterize p athophysiologic correlations.