Fig. 6

Simulation of modified afterloaded contractions with quick muscle restretching (Fig. 5) in the TP + M model. The mode of cardiac muscle contraction is simulated in the following way. The muscle is allowed to contract against different loads (decreased in a from dark to light grey lines) in the same manner as in the isotonic afterloaded mode. Then, at the moments of maximum (end-systolic) shortening (shown by triangles) the muscle is forced to stretch at a velocity much higher than that of its lengthening in the full afterloaded cycle (as shown further for the virtual sample in Fig. 7). Thus, the sample quickly returns to its initial length (L_init = 90%L_max in this simulation) and then relaxes isometrically. Dotted lines are for isometric contraction, dashed lines are for preloaded twitch. a, b Active force normalized to peak isometric force at L_init; length of the virtual sample (in % of L_init). c, d Intracellular Ca²⁺ concentration ([Ca²⁺]_i) and membrane potential