Three-state model predictions of isometric force development and unloaded shortening with bound cross-bridge distributions. (A) Force development predicted by MUSICO (black lines), Duke (green line), and Duke PL (red line) models. Maximum isometric tension To = 300 kPa from mass action kinetic models is scaled to maximum force per myosin filament (MUSICO) Fo = 600 pN. Comparison of bound cross-bridge distributions at 5, 20, 80, and 700 ms are shown as insets where black bars represent weakly bound cross-bridges and red bars the post-power stroke bound myosins (MUSICO). Duke cross-bridge distributions are shown as solid lines (weakly bound as a green line and post-power stroke as a dark green line) and Duke PL as dashed lines (pink and cyan, respectively). Cyan diamonds denote force and time at which the distributions are taken. The evolution of displacement after quick release to unloaded shortening is shown in the inset. (B) The evolution of velocities during isotonic shortening after quick release to zero tension. The velocity transients for the first 10 ms after release are shown in the inset. (C) The cross-bridge distributions and state probability density distribution functions at times tQRel = 5 and 10 ms after the quick release. A complete set of the distributions and distribution functions (at tQRel = 0+, 5, 10, 20, 80, and 200 ms) is shown in Fig. S6.
