Table 1. Baseline Mechanical Variables of Papillary Muscles in Adult and Senescent Rats

Values are mean ± SD.

**P* < 0.05 *vs.* adult rats.

L^{max}= initial length; CSA = cross-sectional area; RF/TF = ratio of resting force to total force; V^{max}= maximum unloaded shortening velocity; ^{max}Vc = maximum shortening velocity; AF = isometric active force normalized per CSA; +dF/dt = peak of the positive force derivative normalized per CSA; ^{max}Vr = maximum lengthening velocity; −dF/dt = peak of the negative force derivative normalized per CSA; R1 =^{max}Vc/^{max}Vr; R2 = (+dF/dt^{max})/ (−dF/dt^{max}).