Figure 1. Mechanical parameters of contraction and relaxation. Top: Muscle shortening length (L/Lmax) plotted versus time. Bottom: Force (F) plotted versus time. Twitch 1 was loaded with preload only at Lmax. Twitch 2 was loaded with the same preload as twitch 1 and was abruptly clamped to zero-load just after the electrical stimulus. Maximum unloaded shortening velocity (Vmax) 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 the lusitropic effect under low load; coefficient R2, the ratio of the peak of positive force derivative (+dF [centered dot] dt sup -1) to the peak of negative force derivative (-dF [centered dot] dt sup -1) tests the lusitropic effect under high load.