Inter-effort hypoxia recovery during high-intensity intermittent exercise enhances oxygen uptake at the onset of efforts while maintaining exercise tolerance

Exercise training in hypoxia enhances physiological adaptations improving exercise performance. However, acute hypoxia generally reduces high-intensity exercise tolerance, limiting its application in sports training. Here, we investigated whether oxygen consumption and exercise tolerance are affected during a session of an emerging high-intensity intermittent training (HIIT) model in hypoxia. This model involves efforts in normoxia with inter-effort hypoxia (IEH) recoveries. Young active males were recruited and completed a graded exercise test in normoxia, followed by HIIT sessions under three conditions: normoxia, continuous normobaric hypoxia (FIO2: ~0.13), and IEH recovery, in different days and random order. Oxygen consumption, ventilatory variables and muscle oxygenation in the vastus laterali were assessed during HIIT sessions consisting of ten 1-min efforts (at 120% of maximal treadmill running speed from the graded test), with 2-min passive recoveries. Compared to normoxia, IEH recovery caused significant hemoglobin desaturation (between 95% and 88%) and a ~ 14% decrease in V̇O2 during recoveries. During efforts, particularly in the first 30 s, VO2 was significantly increased by approximately 7% in the IEH condition compared to normoxia. Notably, exercise task completion was nearly identical between normoxia (87 ± 24%) and IEH recovery conditions (87 ± 18%), but significantly lower in continuous hypoxia (44 ± 27%), along with impaired indexes of O2 metabolism. Additionally, IEH recovery resulted in a significantly lower pulmonary O2 diffusion gradient at a given V̇O2, suggesting a compensatory increase in blood flow. In conclusion, IEH recovery preserved muscle oxygenation and exercise performance while enhancing V̇O2 during efforts.