FIGURE 1.

Conceptual basis of overall cardiac enthalpy output as a function of afterload. Q_B and Q_A define basal and activation heat, respectively. Heat from cross-bridge cycling in isolated cardiac muscle preparations arises from two distinct temporal phases: isometric [Q_X–b(Isom)] and reduction of filament overlap [Q_X–b(short)] corresponding, respectively, to the isovolumic and auxotonic phases of contraction of the heart in vivo. W denotes cross-bridge work. [Modified from Tran et al. (2017) under the aegis of the Rights Link^®-Copyright Clearance Center; Account Number: 3000104389; License Number: 4678431442941].