Physiological Adaptations to Hypoxic vs. Normoxic Training during Intermittent Living High.

In the setting of "living high," it is unclear whether high-intensity interval training (HIIT) should be performed "low" or "high" to stimulate muscular and performance adaptations. Therefore, 10 physically active males participated in a 5-week "live high-train low or high" program (TR), whilst eight subjects were not engaged in any altitude or training intervention (CON). Five days per week (~15.5 h per day), TR was exposed to normobaric hypoxia simulating progressively increasing altitude of ~2,000-3,250 m. Three times per week, TR performed HIIT, administered as unilateral knee-extension training, with one leg in normobaric hypoxia (~4,300 m; TR) and with the other leg in normoxia (TR). "Living high" elicited a consistent elevation in serum erythropoietin concentrations which adequately predicted the increase in hemoglobin mass ( = 0.78, < 0.05; TR: +2.6%, < 0.05; CON: -0.7%, > 0.05). Muscle oxygenation during training was lower in TR vs. TR ( < 0.05). Muscle homogenate buffering capacity and pH-regulating protein abundance were similar between pretest and posttest. Oscillations in muscle blood volume during repeated sprints, as estimated by oscillations in NIRS-derived tHb, increased from pretest to posttest in TR (~80%, < 0.01) but not in TR (~50%, = 0.08). Muscle capillarity (~15%) as well as repeated-sprint ability (~8%) and 3-min maximal performance (~10-15%) increased similarly in both legs ( < 0.05). Maximal isometric strength increased in TR (~8%, < 0.05) but not in TR (~4%, > 0.05). In conclusion, muscular and performance adaptations were largely similar following normoxic vs. hypoxic HIIT. However, hypoxic HIIT stimulated adaptations in isometric strength and muscle perfusion during intermittent sprinting.