Figure 2. Effects of calcium on indices of lusitropy under high or low load (n = 8). Under low load, in contrast to both maximum shortening velocity (sub max VC) and maximum lengthening velocity (sub max Vr), R1 is not significantly modified by major inotropic changes induced by decreasing [Ca²+]o. Under high load, R2 is less modified by major inotropic changes induced by decreasing [Ca²+]o than the peak of the positive force derivative (+dF [centered dot] df sup -1)* and the peak of the negative force derivative (-dF [centered dot] dt sup -1). Data are mean percentages of control values +/- SEM.