All concentrations are spatially averaged and refer to the myoplasmic water volume in the myofibrillar region; except for free [Ca²⁺] and free [Mg²⁺], total concentrations are given. The concentrations are taken from Harwood et al. (2011), except for free [Ca²⁺], fluo-4, and protein (see Model simulations). The concentration of protein applies to the four-state reaction scheme between fluo-4 and protein (Table 2, E). The mean concentration of fluo-4 was estimated from comparisons of fluo-4’s resting fluorescence in swimbladder fibers with the concentration and resting fluorescence of fluo-3 estimated in singly dissected frog twitch fibers (Harkins et al., 1993; Hollingworth et al., 2000, 2001), with appropriate adjustments made for differences in fiber diameters. The fluo-3 concentration estimates in frog fibers are thought to be quite accurate because measurements of both indicator absorbance and fluorescence were made in these fibers (Harkins et al., 1993), and fiber diameters can be measured accurately in frog fibers.