Previous studies have indicated that 13C-MRS in the ultra-high 7T magnetic resonance (MR) field is a potential non-invasive measurement method for assessing changes in muscle glycogen levels in PoD patients. However, in a single study, increases in glycogen intermediates were observed using the even more sensitive 31P-MRS technique in a mouse model of PoD and in glycogen storage disease III in humans. In fact, glycolytic intermediates such as phosphomonoesters (PME), measured by phosphorus-31P-MRS in PoD mouse models, were superior to 13C-MRS in monitoring disease progression and quantifying glycogen, indicating a significant clinical potential of 31P-MRS in humans. It has been shown that 31P-MRS can reliably quantify age- and weight-related differences as well as changes in thyroid function in human muscle metabolism. This study conducted by our institute demonstrates that the technique possesses the necessary sensitivity to measure these subtle muscular metabolic changes. However, there are currently no human 31P-MRS muscle data available for PoD. Therefore, we propose a proof-of-principle study to address this knowledge gap and contribute to establishing a new sensitive muscular biomarker that quantifies the primary disease mechanism, namely glycogen formation, for future longitudinal studies on PoD.