Limitations to oxygen transport and utilization during sprint exercise in humans: evidence for a functional reserve in muscle O2 diffusing capacity

To determine the contribution of convective and diffusive limitations to (V) over dot(O2 peak) during exercise in humans, oxygen transport and haemodynamics were measured in 11 men (22 +/- 2years) during incremental (IE) and 30s all-out cycling sprints (Wingate test, WgT), in normoxia (Nx, P-IO2: 143mmHg) and hypoxia (Hyp, P-IO2: 73mmHg). Carboxyhaemoglobin (COHb) was increased to 6-7% before both WgTs to left-shift the oxyhaemoglobin dissociation curve. Leg (V) over dot(O2 peak) was measured by the Fick method and leg blood flow (BF) with thermodilution, and muscle O-2 diffusing capacity (D-MO2) was calculated. In the WgT mean power output, leg BF, leg O-2 delivery and leg O2 were 7, 5, 28 and 23% lower in Hyp than Nx (P<0.05); however, peak WgT D-MO2 was higher in Hyp (51.5 +/- 9.7) than Nx (20.5 +/- 3.0mlmin(-1)mmHg(-1), P<0.05). Despite a similar P-aO2 (33.3 +/- 2.4 and 34.1 +/- 3.3mmHg), mean capillary P-O2 (16.7 +/- 1.2 and 17.1 +/- 1.6mmHg), and peak perfusion during IE and WgT in Hyp, D-MO2 and leg (V) over dot(O2) were 12 and 14% higher, respectively, during WgT than IE in Hyp (both P<0.05). D-MO2 was insensitive to COHb (COHb: 0.7 vs. 7%, in IE Hyp and WgT Hyp). At exhaustion, the Y equilibration index was well above 1.0 in both conditions, reflecting greater convective than diffusive limitation to the O-2 transfer in both Nx and Hyp. In conclusion, muscle (V) over dot(O2) during sprint exercise is not limited by O-2 delivery, O-2 offloading from haemoglobin or structure-dependent diffusion constraints in the skeletal muscle. These findings reveal a remarkable functional reserve in muscle O-2 diffusing capacity.