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.