Exercise training in normobaric hypoxia in endurance runners. I. Improvement in aerobic performance capacity

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Abstract
This study investigates whether a 6-wk intermittent hypoxia training (IHT), designed to avoid reductions in training loads and intensities, improves the endurance performance capacity of competitive distance runners. Eighteen athletes were randomly assigned to train in normoxia [Nor group; n = 9; maximal oxygen uptake (V̇o2 max) = 61.5 ± 1.1 ml·kg−1·min−1] or intermittently in hypoxia (Hyp group; n = 9; V̇o2 max = 64.2 ± 1.2 ml·kg−1·min−1). Into their usual normoxic training schedule, athletes included two weekly high-intensity (second ventilatory threshold) and moderate-duration (24–40 min) training sessions, performed either in normoxia [inspired O2 fraction (Fi O2) = 20.9%] or in normobaric hypoxia (Fi O2 = 14.5%). Before and after training, all athletes realized 1) a normoxic and hypoxic incremental test to determine V̇o2 max and ventilatory thresholds (first and second ventilatory threshold), and 2) an all-out test at the pretraining minimal velocity eliciting V̇o2 max to determine their time to exhaustion (Tlim) and the parameters of O2 uptake (V̇o2) kinetics. Only the Hyp group significantly improved V̇o2 max (+5% at both Fi O2, P < 0.05), without changes in blood O2-carrying capacity. Moreover, Tlim lengthened in the Hyp group only (+35%, P < 0.001), without significant modifications of V̇o2 kinetics. Despite similar training load, the Nor group displayed no such improvements, with unchanged V̇o2 max (+1%, nonsignificant), Tlim (+10%, nonsignificant), and V̇o2 kinetics. In addition, Tlim improvements in the Hyp group were not correlated with concomitant modifications of other parameters, including V̇o2 max or V̇o2 kinetics. The present IHT model, involving specific high-intensity and moderate-duration hypoxic sessions, may potentialize the metabolic stimuli of training in already trained athletes and elicit peripheral muscle adaptations, resulting in increased endurance performance capacity.