Non-linear regression and curve-fitting may contribute to resolution of the mechanism, summarise information, remove noise, allow speculation regarding unmeasured data, and separate the effects of multiple factors. The isovolumic left ventricular (LV) pressure curve and isometric myocardial tension curve have been curve-fit with polynomial expotential and sinusoidal functions. The isovolumic LV pressure curve and myocardial isometric tension curve are composed of contraction and relaxation processes. We have proposed that hybrid logistic (HL) functions, calculated as the difference between two logistic functions, fit better the isovolumic LV pressure curves at any LV volume, heart rate, and infused calcium (Ca2+) concentration in coronary artery in the excised, cross-circulated canine heart, and the isometric twitch tension curves at any muscle length and extracellular Ca2+ concentration in the ferret right ventricular (RV) papillary muscle. We suggest that the six HL parameters are useful to evaluate the contraction and relaxation processes in the heart and myocardium. This HL approach for the isovolumic LV pressure curves and the isometric twitch tension curves may provide a more useful model for speculating Ca2+ handling, Ca(2+) -Troponin C interaction, and cross-bridge cycling.