The dynamic mechanical behavior of excised rabbit papillary muscles that had been tonically activated by replacing bathing Ca2+with Ba2+was studied. Steady activation was used to visualize the dynamic behavior of cardiac myofilaments more clearly than is possible during twitches, which are complicated by the kinetics of excitation-contraction coupling. To avoid artifacts due to damaged ends of the muscle, the length of a central segment, which was denned by 2 tungsten pins inserted through the muscle, was measured. To test the mechanical behavior of the contractured muscles (at 24°C), the central segment length was sinusoidally oscillated (amplitude 1%) at 15 different frequencies (0.05–30 Hz). The dynamic stiffness of the central segment was calculated from the ratio of force response amplitude to length perturbation amplitude. At low frequencies (below 0.4 Hz), stiffness was approximately constant and reflected the force-length relation. However, in a localized range near 1 Hz, there was a distinct drop in the magnitude of dynamic stiffness to approximately half its low-frequency baseline. This range may reflect the dynamics of attachment and detachment of force generators. The frequency of minimum stiffness was consistent among all muscles (1.3 ± 0.3 Hz). Moreover, no significant change in this frequency was found over the examined range of lengths (90–100% of the segment length that produced maximal developed force) and activation levels (Ba2+ concentration 0.3–1.0 mM). From 2 to 8 Hz, dynamic stiffness appeared to reflect force-velocity properties, but at higher frequencies, another elastic property emerged. At 30 Hz, stiffness was proportional to force, with an apparent series elasticity less than 1.8%. Even though the muscles had only moderate longitudinal inhomogeneity, quantitatively significant (35%) errors would have been introduced had the study relied on total muscle length instead of central segment length.