The Effect of Hyperoxia on Slowed VO2 Kinetics In the Upper Region of Moderate-Intensity Exercise

Abstract

The slower adaptation of pulmonary O2 uptake (VO2) in the upper region of the moderate-intensity exercise (MOD) domain is associated with slower leg blood flow kinetics, suggesting a possible role for O2 delivery in limiting VO2 kinetics in this upper region of MOD. PURPOSE: To examine, in groups of young adults exhibiting either fast or slow VO2 kinetics, the effect of hyperoxic breathing on VO2 kinetics in the upper region of the MOD domain. METHODS: Nine young male adults (age, 27 ± 5 yrs; mean ± SD) completed 4-5 repetitions of leg cycling exercise while breathing either 21% O2 (Normoxia; NORM) or 40% O2 (Hyperoxia; HYP). The protocol consisted of baseline cycling at 20 W, followed by two equal step-transitions (lower step, S1; upper step, S2) up to a work rate corresponding to 90% estimated lactate threshold. VO2 was measured breath-bybreath (mass spectrometer and turbine), and the phase 2 response was modeled to a mono-exponential using non-linear regression. Based on the phase 2 VO2 time constant (τVO2) estimated in S1 (NORM), subjects were grouped according to whether τVO2 was <30s (fast kinetics, FK; n=5) or, τVO2 was >30s (slow kinetics, SK; n=4). RESULTS: In FK, τVO2 was greater (P<0.05) in S2 than S1 during NORM (S2: 33 ± 10 s; S1: 17 ± 7 s) and was not affected by HYP (S2: 33 ± 15 s; S1: 20 ± 7 s). In SK, τVO2 was greater (P<0.05) in S2 than S1 during NORM (S2: 54 ± 18 s; S1: 43 ± 16 s); HYP had no effect on τVO2 in S2 (50 ± 22 s) but was associated with a reduction (P<0.05) in the S1 τVO2 (25 ± 9 s), a value approaching that observed in FK. CONCLUSION: As demonstrated previously, VO2 kinetics were slower in the upper region of MOD in both FK and SK. As kinetics in this upper region did not speed with hyperoxic breathing, this suggests that, in this part of the MOD domain, muscle O2 utilization is limited by factors other than convective and/or diffusive O2 delivery. Supported by NSERC, Canada