Comments on Point:Counterpoint: Hypobaric hypoxia induces/does not induce different responses from normobaric hypoxia.

TO THE EDITOR: Decades of research in exercise physiology have supported the common view that endurance performance suffers most greatly at altitude because oxidative energy production is limited (4). In contrast, Weyand et al. (5) reported that fit males are capable of running just as fast during “all-out” treadmill efforts of 1 min in hypoxic compared with normoxic conditions (13.00% and 20.93% oxygen, respectively), despite a reduction in the aerobic energy available for sprinting. As underlined by Mounier and Brugniaux (2) in their Counterpoint, it is noteworthy that most of our knowledge on this topic is derived from laboratory-based measurements. When comparing hypobaric and normobaric hypoxia, the effect of air resistance should not be neglected. Indeed, the decrease in air density upon ascent to terrestrial (natural) altitude reduces air resistance, which is likely to decrease the energy cost of running at high velocities, without the detrimental effect of reducing energy availability (3). This may explain—to a large extent—why sprinters generally achieve better performances with exposure to natural altitude. Anecdotally, multiple world records were set in the sprint (i.e. 100, 200, and 400 m) disciplines during the Mexico City Olympics held at an altitude of 2,240 m in 1968, whereas no records were set in the middleor long-distance running disciplines. By using a mathematical supply-demand model, Arsac (1) calculated improvements in sprinting (i.e. 60, 100 m) times and changes in the components of the energy cost with changes in altitude from 0 to 4,000 m. It was concluded that as the cost of overcoming air resistance decreases with increasing altitude, better performances may be achieved attributable to more energy being available for acceleration. In summary, it should be underlined that the reduced air resistance with terrestrial altitude (hypobaric hypoxia) is likely to induce different responses than exposure to gas mixtures lowering the oxygen fraction (normobaric hypoxia) during running sprints. Of the few studies modeling the effects of natural altitude on the energetics of sprint performance, to date, no study has directly assessed the impact of a reduced air resistance per se, which warrants further research.