The ventricles of small mammals express mostly MHC, a fast isoform, while the ventricles of large mammals including humans express ~10% MHC on a predominately MHC (slow isoform) background. In failing human ventricles the amount of MHC is dramatically reduced leading to the hypothesis that even small amounts of MHC on a predominately MHC background confer significantly higher rates of force development in healthy ventricles. To test this hypothesis it is necessary to determine the fundamental rate constants of cross-bridge attachment (f_app) and detachment (g_app) for myosins comprised of 100% or MHC which can then be used to calculate twitch time courses for muscles expressing variable ratios of MHC isoforms. In the present study, rat skinned trabeculae expressing either 100% MHC or 100% MHC were used to measure ATPase activity, isometric force, and the rate constant of force redevelopment (k_tr) in solutions of varying [Ca²⁺]. The rate of ATP utilization was approximately 2.5-fold higher in preparations expressing 100% MHC compared to those expressing only MHC, while k_tr was 2-fold faster in MHC myocardium. From these variables, we calculated f_app to be approximately 3-fold higher for than MHC, and g_app to be 2-fold higher in MHC. Mathematical modeling of isometric twitches predicted that small increases in MHC significantly increased the rate of force development (dF/dt_max). These results suggest that low-level expression of MHC has significant effects on contraction kinetics.