Muscle converts chemical free energy from ATP hydrolysis into mechanical work with an efficiency approaching 80% under optimal circumstances. ATP is only an intermediary in energy storage in red muscles. Its concentration remains relatively constant during long exercise, as shown in the NMR studies of Figure 12.14.

The high-energy compound steadily depleted during muscular activity is creatine phosphate (see here). Because the equilibrium for this reaction lies well to the right, virtually all of the muscle adenylate is maintained in the ATP form, rather than as ADP or AMP, as long as creatine phosphate is available. Thus, the energy source in red muscle is creatine phosphate, which regenerates ATP continually as it is depleted by muscle contraction.

Table 8.1 compares the structures and energy fuels in white and red muscle.