We investigated if changing thin filament Ca²⁺ sensitivity alters the rate of contraction, either during normal crossbridge cycling or when crossbridge cycling is increased by inorganic phosphate (Pi). We increased or decreased Ca²⁺ sensitivity of force production by incorporating into rat skinned cardiac trabeculae the troponin C (TnC) mutants V44QTnC^F27W or F20QTnC^F27W, respectively. The rate of isometric contraction was assessed as the rate of force redevelopment (k_tr) after a rapid release and restretch to the original length of the muscle. Both in the absence of added Pi and in the presence of 2.5 mM added Pi: 1) Ca²⁺ sensitivity of k_tr was increased by V44QTnC^F27W and decreased by F20QTnC^F27W compared with control TnC^F27W; 2) k_tr at submaximal Ca²⁺ activation was significantly faster for V44QTnC^F27W and slower for F20QTnC^F27W compared with control TnC^F27W; 3) at maximum Ca²⁺ activation, the k_tr values were similar for control TnC^F27W, V44QTnC^F27W and F20QTnC^F27W and 4) k_tr exhibited a linear dependence on force which was indistinguishable for all TnCs. In the presence of 2.5mM Pi, k_tr was faster at all pCas compared with the values in no added Pi for TnC^F27W, V44QTnC^F27W and F20QTnC^F27W. This study suggests that TnC Ca²⁺ binding properties modulate the rate of cardiac muscle contraction at submaximal levels of Ca²⁺ activation. This result has physiological relevance considering that, on a beat-to-beat basis, the heart contracts at submaximum Ca²⁺ activation.