Divergent functional responses to hyperoxic versus hypoxic high-intensity interval training: a randomized exploratory active-comparator study

Abstract Background Inspired oxygen fraction (FiO₂) may alter the internal physiological cost of supramaximal interval training. This randomized exploratory active-comparator study compared functional adaptations to hyperoxic versus hypoxic Tabata-format high-intensity interval training (HIIT). Methods Twenty-three physically active men completed 3 weeks of supramaximal Tabata-format HIIT (3 sessions·week⁻¹; 8 × 20 s with 10 s recovery) under hyperoxia (FiO₂ = 0.60, n = 13) or hypoxia (FiO₂ = 0.16, n = 10). Intensity was initially prescribed from baseline CPET-derived Wmax and operationally regulated using heart rate to maintain the intended internal cardiovascular strain. V̇O₂max was the primary outcome; ventilatory thresholds, peak blood lactate, perceived exertion, heart-rate outcomes, and available workload records were secondary or exploratory outcomes. Results V̇O₂max increased from 37.77 ± 8.67 to 41.46 ± 8.41 mL·kg⁻¹·min⁻¹ in hyperoxia and from 38.30 ± 9.91 to 39.80 ± 9.80 mL·kg⁻¹·min⁻¹ in hypoxia, with a larger baseline-adjusted improvement under hyperoxia. VT1 increased from 175.0 ± 52.0 to 213.5 ± 46.3 W in hyperoxia and from 160.0 ± 47.4 to 170.0 ± 48.3 W in hypoxia. VT2 increased from 232.7 ± 51.4 to 263.5 ± 40.3 W in hyperoxia and from 207.5 ± 55.3 to 222.5 ± 49.2 W in hypoxia. Peak lactate decreased from 9.58 ± 1.37 to 8.81 ± 1.47 mmol·L⁻¹ in hyperoxia but increased from 8.93 ± 0.84 to 9.25 ± 0.75 mmol·L⁻¹ in hypoxia. Wmax increased from 278.8 ± 55.8 to 334.6 ± 45.1 W in hyperoxia and from 247.5 ± 73.1 to 282.5 ± 63.5 W in hypoxia. Conclusions Hyperoxic and hypoxic Tabata-format HIIT elicited divergent functional response profiles within this internally regulated two-arm protocol. Hyperoxia was associated with broader aerobic functional improvements and lower post-exercise peak lactate, whereas hypoxia produced smaller aerobic gains and a higher post-exercise peak lactate response. These findings should be interpreted as exploratory because of the small sample, absence of a normoxic comparator, and limited session-level external-load and heart-rate recording. Clinical trial number ClinicalTrials.gov, NCT07551869. Retrospectively registered on 5 April 2026.