The aim was to investigate changes in the line width of high-energy phosphates solely originating from the intramyocellular compartment by temporally resolved localised 31P-MRS after aerobic calf muscle exercise. Semi-Laser [2] localised 31P-MRS of gastrocnemius muscle and pulse-acquire-MRS (10cm-surface coil) were performed alternatingley in three exercise bouts separated by >20min inactivity. PCr and Pi were quantified in AMARES from single acquitsitions, with TR=6s. While Pi line width is known to change due to pH, to our knowledge this is the first observation of an exercise induced line width increase of intra-myocellular PCr, which may be associated with myoglobin deoxygenation.