Many current pharmaceutical therapies for systolic heart failure target intracellular [Ca^2 +] ([Ca^2 +]_i) metabolism, or cardiac troponin C (cTnC) on thin filaments, and can have significant side-effects, including arrhythmias or adverse effects on diastolic function. In this study, we tested the feasibility of directly increasing the Ca^2 + binding properties of cTnC to enhance contraction independent of [Ca^2 +]_i in intact cardiomyocytes from healthy and myocardial infarcted (MI) hearts. Specifically, cardiac thin filament activation was enhanced through adenovirus-mediated over-expression of a cardiac troponin C (cTnC) variant designed to have increased Ca^2 + binding affinity conferred by single amino acid substitution (L48Q). In skinned cardiac trabeculae and myofibrils we and others have shown that substitution of L48Q cTnC for native cTnC increases Ca^2 + sensitivity of force and the maximal rate of force development. Here we introduced L48Q cTnC into myofilaments of intact cardiomyocytes via adeno-viral transduction to deliver cDNA for the mutant or wild type (WT) cTnC protein. Using video-microscopy to monitor cell contraction, relaxation, and intracellular Ca^2 + transients (Fura-2), we report that incorporation of L48Q cTnC significantly increased contractility of cardiomyocytes from healthy and MI hearts without adversely affecting Ca^2 + transient properties or relaxation. The improvements in contractility from L48Q cTnC expression are likely the result of enhanced contractile efficiency, as intracellular Ca^2 + transient amplitudes were not affected. Expression and incorporation of L48Q cTnC into myofilaments was confirmed by Western blot analysis of myofibrils from transduced cardiomyocytes, which indicated replacement of 18 ± 2% of native cTnC with L48Q cTnC. These experiments demonstrate the feasibility of directly targeting cardiac thin filament proteins to enhance cardiomyocyte contractility that is impaired following MI.