Unloaded twitch contraction. A simulation fitted to the half-sarcomere length trace measured at 37°C using an enzymatically isolated rat cardiac myocyte. Model parameters are listed in Table S4. (A) The two-state model was driven by a Ca²⁺ transient measured in relative units using Fura-2 fluorescence and scaled to the range 0.1–1.0 µM for the calculations. (B) The proportion of binding sites that are available (solid lines), bound (dashed lines), and that could potentially be activated (dotted lines, calculated as described in Eq. 3 of Campbell, 2009). (C) Predicted half-sarcomere length record. (D) Predicted half-sarcomere length (black) overlaid on the mean experimental record (dark gray). The shaded region shows the standard error of the experimental recordings (10 consecutive twitches from a single cell).