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

¹Service de Physiologie Clinique et des Explorations Fonctionnelles Respiratoires et de l'Exercice, Département de Physiologie, Équipe d'Accueil 3072, Strasbourg, France; ²Institute of Anatomy, University of Bern, Bern, Switzerland; ³Service de Cardiologie, Hôpitaux Civils de Colmar, Colmar, France; and ⁴U-446 Institut National de la Santé et de la Recherche Médicale, Faculté de Pharmacie, Châtenay-Malabry, France

Submitted 29 March 2005 ; accepted in final form 1 December 2005

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 (O₂) and speed at VT₂, maximal O₂ (O_{2 max}), and time to exhaustion at velocity of O_{2 max} (minimal speed associated with O_{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, O₂ at VT₂, and O_{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.

intermittent hypoxia training; skeletal muscle; mitochondria; time to exhaustion; endurance athletes

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