["The Journal of Physiology, EarlyView. ", "\nAbstract figure legend This study used interleaved, non‐invasive 31P and 1H magnetic resonance spectroscopy (MRS) to test hypotheses related to muscle oxidative capacity, intramyocellular oxygen tension (PO2${{P}_{{{{\\mathrm{O}}}_2}}}$), critical PO2${{P}_{{{{\\mathrm{O}}}_2}}}$ and mitochondrial coupling (P/O) in the vastus lateralis muscle of 14 young and 10 older adults. Measurements were taken during 24 s of muscle contractions followed by 10 min of recovery and 8 min of circulatory occlusion. Despite the expected lower muscle oxidative capacity in the older group, there was no age‐related difference in oxygen availability, critical PO2${{P}_{{{{\\mathrm{O}}}_2}}}$ or P/O. Thus, these results do not support a primary role of limited oxygen availability or impaired mitochondrial coupling in the lower oxidative capacity of older compared with young vastus lateralis muscle. Abbreviation: PCr, phosphocreatine.\n\n\n\n\n\n\n\n\n\nAbstract\nSkeletal muscle oxidative capacity is a useful in vivo marker of mitochondrial health and is generally lower in the knee‐extensor muscles of older compared with younger adults. The causes of this lower oxidative capacity in older muscle are unclear. We used magnetic resonance spectroscopy to investigate the influence of intramyocellular oxygen availability and the coupling (P/O ratio) between mitochondrial respiration and ATP production on oxidative capacity in the knee‐extensor muscles of 14 young and 10 older adults. Participants completed a 24‐s contraction protocol followed by 10 min of recovery and 8 min of cuff occlusion while interleaved 31P and 1H spectroscopy data were acquired. Oxidative capacity was calculated as the rate constant of phosphocreatine recovery, and intramyocellular oxygen tension (PO2${{P}_{{{{\\mathrm{O}}}_2}}}$) was determined from the deoxymyoglobin signal. Critical PO2${{P}_{{{{\\mathrm{O}}}_2}}}$, the point at which respiration is limited owing to insufficient oxygen availability, and the P/O ratio were determined for each individual. Oxidative capacity was lower in older than younger muscles, whereas neither critical PO2${{P}_{{{{\\mathrm{O}}}_2}}}$ nor the P/O ratio differed between groups. On average, PO2${{P}_{{{{\\mathrm{O}}}_2}}}$ remained above the critical PO2${{P}_{{{{\\mathrm{O}}}_2}}}$ throughout the protocol in both young and older muscle. Oxidative capacity was modestly related to PO2${{P}_{{{{\\mathrm{O}}}_2}}}$ during recovery in young but not older muscle, and mitochondrial coupling was unrelated to oxidative capacity. These novel results do not support a primary role for limited oxygen availability or impaired mitochondrial coupling in the lower oxidative capacity of older knee‐extensor muscles and suggest instead that other mechanisms, such as lower mitochondrial content, might be responsible.\n\n\n\n\n\n\n\n\n\nKey points\n\nKnee‐extensor muscle oxidative capacity is generally lower in older age, but the questions of whether this decline is attributable, in part, to insufficient oxygen availability or altered coupling between mitochondrial energy production and oxygen consumption remain open.\nInterleaved 31P and 1H magnetic resonance spectroscopy was used to measure intramyocellular phosphocreatine and deoxygenated myoglobin in vivo, respectively, in the knee‐extensor muscles of young and older adults in response to a 24‐s contraction protocol and 8 min of circulatory occlusion.\nOxidative capacity was indeed lower in the older muscles, but intracellular oxygen availability during and after contractions was sufficient to support oxidative metabolism and did not differ in young and older muscles.\nMitochondrial coupling did not differ by age and was unrelated to oxidative capacity.\nThus, the lower knee‐extensor muscle oxidative capacity in older age is not a result of inadequate intramyocellular oxygen availability or differences in mitochondrial coupling.\n\n\n"]