Fig. 1. Mechanical parameters of contraction and relaxation. (Top ) Muscle shortening length (L/L^max) plotted versus  time. (Bottom ) Force (mN/mm²) plotted versus  time. Tetanus 1 was isotonic and loaded with preload only to determine the maximum extent of shortening (ΔL), maximum shortening velocity (Vc), and maximum lengthening velocity (Vr). Tetanus 2 was loaded with preload and was abruptly clamped to zero-load immediately after the electrical stimulus to determine the maximum unloaded shortening velocity (V^max). Tetanus 3 was fully isometric at L^maxto determine the maximum active force (AF) and the peak of the positive (+dF/dt) and negative (−dF/dt) force derivatives.