Evidence that myoglobin PO2 is a non-invasive measure of in vivo skeletal muscle mitochondrial PO2 during exercise in humans.

Muscle mitochondrial partial pressure of oxygen (PmitoO2) is a key determinant of skeletal muscle oxygen consumption (V̇O2), metabolism, gene regulation, and adaptation in health and disease. Yet PmitoO2 remains difficult to measure directly in vivo in humans during exercise. Myoglobin-associated PO2 (PMbO2), measured non-invasively using proton magnetic resonance spectroscopy, may provide a measure of PmitoO2 during maximal exercise in humans because Mb is coupled to mitochondrial oxygen availability and PMbO2 falls in the low PO2 range expected for PmitoO2 in vivo. However, it remains unknown whether PMbO2 during maximal exercise varies systematically with muscle V̇O2max, as would be expected if PMbO2 reflects PmitoO2 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 (FIO2), we measured PMbO2 using myoglobin desaturation measured by proton magnetic resonance spectroscopy and muscle V̇O2max from leg blood flow and the arterial-femoral venous O2 content difference. Across FIO2 conditions, muscle V̇O2max varied systematically with PMbO2 both before and after training, consistent with O2-dependent mitochondrial respiration. When interpreted within a canonical hyperbolic framework, the V̇O2-PMbO2 relationship yielded low apparent mitochondrial P50 values that were broadly consistent with values reported for human skeletal muscle isolated mitochondria studied in vitro. A complementary linear P50 analysis that did not assume a hyperbolic relationship supported the same interpretation. Together, these findings provide data-supported, proof-of-concept evidence that PMbO2 reflects in vivo PmitoO2 during maximal exercise in humans.