FIGURE 3.

Representative ³¹P MR spectra acquired using four-angle saturation transfer (FAST) experiment with FA = 52° during exercise in the gastrocnemius medialis muscle of young and senior participants. Spectra with γ-ATP saturation (-2.48 ppm) are drawn as a red line, and the controlled spectra are drawn as a gray line. The controlled spectra are a combination of two control saturations connected at 2 ppm, i.e., the control experiment for the PCr-to-ATP exchange rate at a downfield frequency mirrored around PCr at 2.48 ppm, and with the control experiment for the Pi-to-ATP exchange rate at a downfield frequency mirrored around the Pi resonance at 12.52 ppm. Please note the drop in PCr and Pi signal caused by the chemical exchange.