Figure 2. Dynamic measures of skeletal muscle high-energy phosphates.

Representative ³¹P-MR spectra at baseline (A) and at fatigue (B) of a healthy middle-aged subject. Time course of skeletal muscle high-energy phosphate metabolites: PCr (dark blue) and ATP (light blue) and inorganic phosphate (red) before, during, and after exercise in a healthy middle-aged (C) and frail older subject (D). The rate of PCr decline during exercise is several-fold faster in the frail older subject.