In Vivo Quantification of Mitochondrial O 2 Affinity in Human Skeletal Muscle Using 1H-MR Spectroscopy of Deoxy-Myoglobin

Mitochondrial oxygen (O 2 ) affinity is a fundamental determinant of oxidative phosphorylation capacity, which has yet to be directly measured in human skeletal muscle in vivo. Purpose: To directly and non-invasively determine the apparent mitochondrial O 2 affinity (i.e., P50) in human skeletal muscle. Method: We used proton magnetic resonance spectroscopy (1H-MRS) of deoxymyoglobin (deoxy-Mb) on a 3T system during a circulatory occlusion of the lower limb to simultaneously quantify intracellular partial pressure of O 2 (iPO 2 ) and resting tissue-specific O 2 consumption (VO 2 ) in the gastrocnemius muscle of sedentary young adults (n = 11). Result: Under these resting conditions, the VO 2 -iPO 2 relationship conformed to a Michaelis-Menten kinetics model (goodness of fit: r2 = 0.84 ± 0.12). The estimated Vmax, reflecting basal metabolic rate, reached 0.20 ± 0.06 mM·min-1, and the apparent mitochondrial P50 was 0.74 ± 0.55 mmHg. Vmax and P50 were positively correlated (r = 0.85, p = 0.0010). This strong correlation remained when data were log-transformed (r = 0.83, p = 0.0017). Conclusion: These results, obtained in human muscles in vivo, demonstrate that mitochondrial respiration exhibits a very high O 2 affinity (P50 ≈ 0.7 mmHg). In addition, it quantitatively identifies the range across which oxidative phosphorylation in the skeletal muscle is O 2 -independent (resting iPO 2 under physiological conditions) or becomes O 2 -sensitive (severe exercise and hypoxia). Moreover, the finding that muscle respiratory flux was associated with the apparent mitochondrial O 2 affinity suggests additional regulatory mechanisms within the respiratory chain to fine-tune oxidative phosphorylation to muscle ATP demand even at rest. This study provides a robust quantitative framework for interpreting in vivo respiratory control in both health and disease. Funding: This work was funded by the National Institute of Health Grant R01AG089307. This abstract was presented at the American Physiology Summit 2026 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.