As the left ventricular (LV) pressure curve and myocardial tension curve in heart are composed of contraction and relaxation processes, we have found that hybrid logistic (HL) function calculated as the difference between two logistic functions curve-fits better the isovolumic LV pressure curve and the isometric twitch tension curve than the conventional polynomial exponential and sinusoidal functions. Increase and decrease in intracellular Ca2+ concentration regulate myocardial contraction and relaxation. Recently, we reported that intracellular Ca2+ transient (CaT) curves measured using the calcium-sensitive bioluminescent protein, aequorin, were better curve-fitted by HL function compared to the polynomial exponential function in the isolated rabbit RV and mouse LV papillary muscles. We speculate that the first logistic component curve of HL fit represents the concentration of the Ca2+ inflow into the cytoplasmic space, the concentration of Ca2+ released from sarcoplasmic reticulum (SR), the concentration of Ca2+ binding to troponin C (TnC), and the attached number of cross-bridge (CB) and their time courses, and that the second logistic component curve of HL fit represents the concentration of Ca2+ sequestered into SR, the concentration of Ca2+ removal from the cytoplasmic space, the concentration of Ca2+ released from TnC, and the detached number of CB and their time courses. This HL approach for CaT curve may provide a more useful model for investigating Ca2+ handling, Ca(2+) -TnC interaction, and CB cycling.