Alterations in T1 have implications on the physiological interpretation of dynamic high-energy phosphate data. Progressive saturation during rest-exercise-recovery experiments was used to simultaneously quantify PCr depletion, recovery and T1 values. The data were acquired using localised 31P MRS in human gastrocnemius. Apparent T1 values increased from 5.0 ± 0.4 s at rest to an average value of 5.8 ± 0.5 s during exercise, with a peak in early exercise and returning towards baseline values before exercise was ceased. These alterations may be explained by changes in the chemical exchange rates of PCr with ATP and Pi.