Evidence that myoglobin PO 2 is a non-invasive measure of in vivo skeletal muscle mitochondrial PO 2 during exercise in humans

Muscle mitochondrial partial pressure of oxygen (P mito O 2 ) is a key determinant of skeletal muscle oxygen consumption (V̇O 2 ), metabolism, gene regulation, and adaptation in health and disease. Yet P mito O 2 remains difficult to measure directly in vivo in humans during exercise. Myoglobin-associated PO 2 (P Mb O 2 ), measured non-invasively using proton magnetic resonance spectroscopy, may provide a measure of P mito O 2 during maximal exercise in humans because Mb is coupled to mitochondrial oxygen availability and P Mb O 2 falls in the low PO 2 range expected for P mito O 2 in vivo. However, it remains unknown whether P Mb O 2 during maximal exercise varies systematically with muscle V̇O 2max , as would be expected if P Mb O 2 reflects P mito O 2 in vivo. We analyzed data from five initially sedentary males before and after 8 weeks of single-leg knee-extensor exercise (KE) training. During maximal KE under 0.12, 0.21, and 1.00 fractions of inspired oxygen (F I O 2 ), we measured P Mb O 2 using myoglobin desaturation measured by proton magnetic resonance spectroscopy and muscle V̇O 2max from leg blood flow and the arterial–femoral venous O 2 content difference. Across F I O 2 conditions, muscle V̇O 2max varied systematically with P Mb O 2 both before and after training, consistent with O 2 -dependent mitochondrial respiration. When interpreted within a canonical hyperbolic framework, the V̇O 2 –P Mb O 2 relationship yielded low apparent mitochondrial P 50 values that were broadly consistent with values reported for human skeletal muscle isolated mitochondria studied in vitro. A complementary linear P 50 analysis that did not assume a hyperbolic relationship supported the same interpretation. Together, these findings provide data-supported, proof-of-concept evidence that P Mb O 2 reflects in vivo P mito O 2 during maximal exercise in humans.