A normal heart increases its contractile force with increasing heart rate. Although calcium handling and myofibrillar proteins have been implicated in maintaining this positive force-frequency relationship (FFR), the exact mechanisms by which it occurs have not been addressed. In this study, we have developed an analytical method to define the calcium-force loop data, which characterizes the function of the contractile proteins in response to calcium that is independent of the calcium handling proteins. Results demonstrate that increasing the stimulation frequency causes increased force production per unit calcium concentration and decreased frequency-dependent calcium sensitivity during the relaxation phase. We hypothesize that phosphorylation of myosin binding protein-C (MyBP-C) and troponin I (TnI) acts coordinately to change the rates of force generation and relaxation, respectively. To test this hypothesis, we performed simultaneous calcium and force measurements on stimulated intact mouse papillary bundles before and after inhibition of MyBP-C and TnI phosphorylation using the calcium calmodulin kinase II (CaMK2) inhibitor, autocamtide inhibitory peptide, or the protein kinase A (PKA) inhibitor, 14-22 amide. CaMK2 inhibition reduced both MyBP-C and TnI phosphorylation and decreased active force without changing the magnitude of the [Ca2+] transient. This reduced the normalized change in force per change in calcium by 19-39%. Data analyses demonstrated that CaMK2 inhibition changed the myofilament characteristics via a crossbridge feedback mechanism. These results strongly suggest that the phosphorylation of MyBP-C and TnI contributes significantly to the rates of force development and relaxation.