The series of drops in PO2 which comprise the oxygen cascade from the air to mammalian tissue can provide useful information about O2 transport during exercise in both health and disease, but the complete cascade has been challenging to study in vivo. This paper reviews a series of in vivo human knee-extensor studies which focus on the determinants of maximal O2 consumption (˙VO2max) in exercising muscle and concludes with a characterization of the complete O2 cascade in maximally exercising human muscle. Specifically, three issues have been addressed: 1) determinants of O2 extraction under conditions of very high muscle blood flow; 2) the role of O2 diffusivity in determining the maximum O2 flux rate(˙VO2max); and 3) myoglobin associated PO2 as a indicator of O2 transport and cellular respiration rate. In summary, these investigations demonstrate that in humans O2 extraction can be uncompromised despite high mass specific blood flows, perhaps in part because of an increased capillary density in exercise trained subjects. Exercise in hypoxia reduces ˙VO2max, but as calculated diffusability of O2 from blood to muscle is constant this suggests that a fixed O2 diffusivity plays a key role in limiting maximal O2 uptake. Supporting evidence of a substantial PO2 gradient from blood to myoglobin also suggests a resistance to the diffusion of O2 between red cell and sarcolemma, which may be present even at submaximal exercise. Finally, the proportionate relationship between myoglobin associated PO2 and ˙VO2max in conditions of normoxia and hypoxia additionally supports the hypothesis that maximal respiratory rate of muscle cells is limited by O2 supply.