Phosphorus nuclear magnetic resonance: a non-invasive technique for the study of muscle bioenergetics during exercise

Phosphorus nuclear magnetic resonance (31P NMR) spectroscopy is a non-destructive analytical laboratory technique that, due to recent technical advances, has become applicable to the study of high-energy phosphate metabolism in both animal and human extremity muscles (in vivo). 31P NMR can assay cellular phosphocreatine, ATP, inorganic phosphate, the phosphorylated glycolytic intermediates, and intra-cellular pH in either resting or exercising muscle, in a non-invasive manner. NMR uses non-perturbing levels of radio-frequency energy as its biophysical probe and can therefore safely study intact muscle in a repeated fashion while exerting no artifactual influence on ongoing metabolic processes. Compared with standard tissue biopsy and biochemical assay techniques, NMR possesses the advantages of being non-invasive, allowing serial in situ studies of the same tissue sample, and providing measurements of only active (unbound) metabolites. NMR studies of exercising muscle have yielded information regarding fatigue mechanisms at the cellular level and are helping resolve long-standing questions regarding the metabolic control of glycolysis, oxidative phosphorylation, and post-exercise phosphocreatine re-synthesis. NMR is also being utilized to measure enzymatic reaction rates in vivo. In the near future, other forms of NMR spectroscopy may also permit the non-invasive measurement of tissue glycogen and lactate content.