Effect of hyperoxia on critical power and v o 2 kinetics during 1 upright cycling 2

25 Introduction/Purpose: Critical power (CP) is a fundamental parameter defining high26 intensity exercise tolerance, however its physiological determinants are incompletely 27 understood. The present study determined the impact of hyperoxia on CP, the time constant 28 of phase II pulmonary oxygen uptake kinetics (τ!O2), and muscle oxygenation (assessed by 29 near-infrared spectroscopy) in 9 healthy men performing upright cycle ergometry. Methods: 30 CP was determined in normoxia and hyperoxia (fraction of inspired O2 = 0.5) via 4 severe31 intensity constant load exercise tests to exhaustion on a cycle ergometer, repeated once in 32 each condition. During each test, τ!O2 and the time constant of muscle deoxyhaemoglobin 33 kinetics (τ[HHb]), alongside absolute concentrations of muscle oxyhaemoglobin ([HbO2]), 34 were determined. Results: CP was greater (hyperoxia: 216 ± 30 vs. normoxia: 197 ± 29W; P 35 < 0.001) whereas W’ was reduced (hyperoxia: 15.4 ± 5.2 kJ, normoxia: 17.5 ± 4.3 W; P = 36 0.037) in hyperoxia compared to normoxia. τ!O2 (hyperoxia: 35 ± 12 vs normoxia: 33 ± 10 s; 37 P = 0.33) and τ[HHb] (hyperoxia: 11 ± 5 vs. normoxia: 14 ± 5 s; P = 0.65) were unchanged 38 between conditions, whereas [HbO2] during exercise was greater in hyperoxia compared to 39 normoxia (hyperoxia: 73 ± 20 vs. normoxia: 66 ± 15 μM; P = 0.001). Conclusion: This study 40 provides novel insights into the physiological determinants of CP and by extension, exercise 41 tolerance. Microvascular oxygenation and CP were improved during exercise in hyperoxia 42 compared with normoxia. Importantly, the improved microvascular oxygenation afforded by 43 hyperoxia did not alter τ!O2, suggesting that microvascular O2 availability is an independent 44 determinant of the upper limit for steady-state exercise, i.e. CP. 45