Exercise training in normobaric hypoxia in endurance runners. II. Improvement of mitochondrial properties in skeletal muscle

This study investigates whether adaptations of mitochondrial function accompany the improvement of endurance performance capacity observed in well-trained athletes after an intermittent hypoxic training program. Fifteen endurance-trained athletes performed two weekly training sessions on treadmill at the velocity associated with the second ventilatory threshold (VT₂) with inspired O₂ fraction = 14.5% [hypoxic group (Hyp), n = 8] or with inspired O₂ fraction = 21% [normoxic group (Nor), n = 7], integrated into their usual training, for 6 wk. Before and after training, oxygen uptake (V̇O₂) and speed at VT₂, maximal V̇O₂ (V̇O_{2 max}), and time to exhaustion at velocity of V̇O_{2 max} (minimal speed associated with V̇O_{2 max}) were measured, and muscle biopsies of vastus lateralis were harvested. Muscle oxidative capacities and sensitivity of mitochondrial respiration to ADP (K_m) were evaluated on permeabilized muscle fibers. Time to exhaustion, V̇O₂ at VT₂, and V̇O_{2 max} were significantly improved in Hyp (+42, +8, and +5%, respectively) but not in Nor. No increase in muscle oxidative capacity was obtained with either training protocol. However, mitochondrial regulation shifted to a more oxidative profile in Hyp only as shown by the increased K_m for ADP (Nor: before 476 ± 63, after 524 ± 62 μM, not significant; Hyp: before 441 ± 59, after 694 ± 51 μM, P < 0.05). Thus including hypoxia sessions into the usual training of athletes qualitatively ameliorates mitochondrial function by increasing the respiratory control by creatine, providing a tighter integration between ATP demand and supply.