Calcium binding cooperativity measures calculated by the models with the different number of interacting cross-bridges.

<p>Here, the steady state solution of models is shown depending on calcium concentration with concentration given in arbitrary units at half-sarcomere length of 1.05 <i>μ</i>m. The model solutions are designated by colors with green and blue depending on whether the parameters were found by fitting the data using <i>q</i> = 3 or <i>q</i> = 1 model, respectively; and by dashed, solid and line with dots, depending on whether the simulations were done using <i>q</i> = 1, <i>q</i> = 3 or <i>q</i> = 4 model, respectively. A: Normalized force-calcium relationship found by the model. All solutions demonstrate the low cooperativity indicating that this aspect of the model has to be refined. B: The Hill coefficient calculated from the slope of force-calcium relationship in log-log scale. For non-cooperative binding, the slope is expected to be 1 (black dashed line). In addition to the solutions found for optimized model parameters, we calculated the Hill coefficient for model solutions with the larger free energy changes induced by tropomyosin deformation (the free energies of the segments are given in the figure legend). Note how the increase in tropomyosin free energy changes induced by cross-bridges and calcium binding leads to the increase of the Hill coefficient in the models with cooperativity effects included (<i>q</i> = 3 and 4) in contrast to the model with only one cross-bridge that can change its state (<i>q</i> = 1).