The amount of work the heart can perform during ejection is governed by the inherent contractile properties of individual myocytes. One way to alter contractile properties is to alter contractile proteins such as myosin heavy chain (MyHC), which is known to demonstrate isoform plasticity in response to disease states. The purpose of this study was to examine myocyte functionality over the complete range of MyHC expression in the heart, from 100% -MyHC to 100% -MyHC using euthyroid and hypothyroid rats. Peak power output in skinned cardiac myocytes decreased as a nearly linear function of -MyHC expression during maximal (r² = 0.82, n = 44) and submaximal (r² = 0.82, n = 31) Ca²⁺ activation. To determine if single myocyte function translated to the level of the whole heart, power output was measured in working heart preparations expressing varied ratios of MyHC. Left ventricular power output of isolated working heart preparations also decreased as a linear function of increasing -MyHC expression (r² = 0.82, n = 34). These results demonstrate that power output is highly dependent on MyHC expression in single myocytes and this translates to the performance of working left ventricles.