Plot of TTP versus RT50. Values were calculated as the weighted averages from all three experimental protocols (length, frequency, and β-adrenergic stimulation). Regardless of the experimental group to which the animals belong, TTP (as a surrogate for contraction) and RT50 (as a surrogate for relaxation) remain tightly correlated, with an overall Pearson coefficient of 0.943. These results demonstrate that CR coupling persists when contraction and relaxation kinetics are determined from contraction and relaxation times and not a scaling of relaxation with peak force.