Population of different cross-bridge states in simulations using one-site and three-site models under different conditions. (A) Cross-bridge distributions in isometric contraction assuming three sites per target zone (each with identical myosin affinity) and linear cross-bridge elasticity. (B) Cross-bridge distributions in A shifted so that the distributions for the three sites overlap (dark lines: dashed [left site], full [middle], and dotted [right]). These distributions are also superimposed on cross-bridge distributions for one-site model (bright thick lines). (C) Cross-bridge distributions during steady-state shortening at 4,000 nm/s for otherwise similar conditions as in A. (D) Same type of plot as in B but for velocity of 4,000 nm/s (as in C). (E) Cross-bridge distributions in isometric contraction assuming one or three sites per target zone (with similar actin affinity) and nonlinear cross-bridge elasticity as in Fig. S1. Distributions are plotted as in B. (F) Cross-bridge distributions for shortening at 4,000 nm/s assuming one or three sites per target zone and nonlinear cross-bridge elasticity as in Fig. S1. (G) Cross-bridge distributions for isometric contraction assuming three sites per target zone and linear cross-bridge elasticity but 2.7 k_BT higher free energy (lower affinity) of two peripheral sites compared with the central site. In all other panels, the minimum free-energy levels are assumed identical for all sites. Color coding as in A in all panels. Note the appreciable similarity between distributions for one and three sites when plotted as in B and D–F.