Geometry of the multicompartment model used in the simulations. The myoplasm of a half-sarcomere of one myofibril is divided into 18 equal-volume compartments (6 longitudinal by 3 radial); “z” and “m” denote the locations of the z-line and m-line. SR Ca²⁺ release enters the compartment indicated by the large downward arrows, which differs according to fiber type. Ca²⁺ pumping takes place in the six outermost compartments (small upward arrows). Troponin is restricted to the nine compartments located within 1 µm of the z-line, the approximate length of the thin filament. The remaining constituents of the model (ATP, parvalbumin, and furaptra) have access to all compartments by diffusion. In the mouse simulations, the radius of the sarcomere (0.375 µm) is slightly smaller than depicted, a difference that, by itself, results in only very small differences in the calculations.