The Energy Cost Of Treadmill Running In Normobaric Hypoxia: The Energy Equivalent Of Oxygen Uptake Matters!

The energy cost of running (Erun) is a key determinant of endurance running performance. Erun appears to be lower in hypoxia (HYPO) compared to normoxia (NORM); however, this observation may be a result of expressing Erun as a steady-state V̇O2 at a common speed, which potentially ignores the difference in relative intensity and the energy equivalent of V̇O2 in HYPO. The expression of Erun as an energy cost between normobaric NORM and HYPO has not yet been evaluated. PURPOSE: To compare Erun during submaximal treadmill running in NORM and HYPO expressed as a V̇O2 (L·min-1) and its energy equivalent (J·kg-1·m-1). METHODS: 15 male and female runners (29 ± 8 years, 179 ± 9 cm, 76 ± 16 kg, V̇O2max = 45.5 ± 6.7 ml·kg-1·min-1) ran for 10 minutes at 70, 80 and 90% of lactate threshold speed in two, single-blind and randomized conditions: NORM (Pb = 668 ± 3 mmHg, PO2 = 140 ± 1 mmHg and HYPO (~2300 m, FIO2 = 18.3 ± 0.1%, PO2 = 122 ± 0.5 mmHg), respectively. Erun was calculated from steady-state V̇O2 and V̇CO2 and expressed as V̇O2 (L·min-1) and energy cost (J·kg-1·m-1), respectively. The oxygen cost of ventilation was estimated from V̇E. V̇O2 and Erun were compared across conditions. RESULTS: V̇E (HYPO: 59.4 ± 3.0 L·min-1 vs NORM: 55.8 ± 3.0 L·min-1, p = 0.009, η2 = 0.05) ) and RER (HYPO: 0.89 ± 0.01 vs. NORM: 0.86 ± 0.01, p = 0.02, η2 = 0.14) were significantly higher in HYPO, resulting in a ~ 4% greater energy equivalent of V̇O2 in HYPO. When accounting for the extra cost of ventilation during running in HYPO, V̇O2 was significantly lower in HYPO (2.37 ± 0.13 L·min-1) compared to NORM (2.54 ± 0.13 L·min-1 p = 0.002, η2 = 0.10). No effect of altering FIO2 was observed for Erun across speeds (HYPO: 3.71 ± 0.18 J·kg-1·m-1 vs. NORM: 3.80 ± 0.18 J·kg-1·m-1, p = 0.17, η2 = 0.06). CONCLUSION: These data highlight the importance of assessing Erun as an energy cost per unit distance (e.g., J·kg-1·m-1) rather than a V̇O2 (i.e., in L·min-1 or ml·kg-1·min-1); the energy cost of running may incorrectly be interpreted as lower in HYPO when expressed only as V̇O2 which has important training and performance prescription implications, particularly when relative intensity (and thus the energy equivalent of V̇O2) differs, for example between HYPO and NORM shown here. This study was supported by the Turing Scheme (UK) and NSERC (Canada)