Isometric force and ATPase activity. Multicellular trabecula-like skinned muscle preparations were isolated from frozen samples by mechanical dissociation (Witayavanitkul et al., 2014). Force and simultaneous ATPase activity were measured by an enzyme-coupled assay (n = 8 hearts per group; approximately three preparations/heart; SL = 2.2 μm; 25°C). (A) Average force–[Ca²⁺] relationship in Sham (open symbols) and AOB (closed symbols) groups. (B) Average force–ATPase activity relationship in Sham and AOB multicellular preparations. Force is normalized to cross-sectional area and ATPase activity is normalized to muscle volume (length times cross-sectional area). The slope of this relationship is termed tension-cost, the magnitude of force dependent cross-bridge ATP consumption rate, which is inversely proportional to cross-bridge detachment rate. Approximately three muscles were studied per heart. Subsequently, multiple muscle data were averaged to derive a single average datum per heart.