Figure 1. Mechanical parameters of contraction and relaxation. (Upper) Muscle shortening length (L/L^max) plotted versus time. (Lower) Force (F) plotted versus time. Twitch 1 was loaded with preload only at L^max. Twitch 2 was loaded with the same preload as twitch 1 but was abruptly clamped to zero-load with critical damping just after electrical stimulus; maximum unloaded shortening velocity (V^max) was determined from this twitch. Twitch 3 was fully isometric. Coefficient R1, the ratio of maximum shortening velocity (sub max Vc) to maximum lengthening velocity (sub max Vr), tests lusitropy under low load; coefficient R2, the ratio of the peak of the positive force derivative (+dF [centered dot] dt sup -1) to the peak of the negative force derivative (-dF [centered dot] dt sup -1), tests lusitropy under high load.