Impact Of Heat Acclimation On Steady-State Exercise And Time-Trial Performance In Hypobaric Hypoxia (3500m)

Abstract

Heat acclimation (HA) has been reported to improve endurance exercise performance in normobaric hypoxia. However, the impact of prior HA on exercise performance in hypobaric hypoxia (HH) is unclear. PURPOSE: To determine whether HA alters steady-state (SS) exercise responses and time-trial (TT) cycle performance during a 30 hour exposure to HH. METHODS: Thirteen sea level (SL) resident men (mean ± SD; age: 21 ± 3 years; height: 173 ± 8 cm; weight: 75 ± 12 kg; SL cycle ergometer VO2peak: 43 ± 5 ml·kg-1·min-1) participated in two 30 hour HH exposures in a hypobaric chamber (496mmHg or ~3500m, 20°C, 20% RH). The HH exposures were separated by a 14 day washout period during which volunteers completed an 8-day exercise-HA protocol (2 hours of treadmill walking: 5km·h-1, 2% grade; 40°C, 40% RH). During each HH exposure, volunteers completed 30 min of SS exercise followed by a 15 min cycle TT at ~2 and 24 hours of exposure. SS exercise consisted of cycling at ~50% SL VO2peak while gas exchange (oxygen consumption (VO2), minute ventilation (VE), end tidal partial pressure of oxygen (PETO2)), and oxygen saturation (SpO2) were recorded. For the TT, volunteers completed as much work (kJ) as possible in 15 min, with heart rate (HR), SpO2, and Rating of Perceived Exertion (RPE, Borg Scale) recorded at the end of exercise. A two way repeated measures ANOVA was used to compare the effects of condition (Pre HA and Post HA) and time (~2 and 24 hours). RESULTS: There were no condition x time interaction effects among the SS or TT variables. For main effect of condition, from Pre HA vs. Post HA, SS VO2 tended to be lower (1.63 ± 0.00 L·min-1 vs. 1.60 ± 0.01 L·min-1; p = 0.05), PETO2 was higher (75.7 ± 1.0 mmHg vs. 76.8 ± 0.7 mmHg; p < 0.01), and VE and SpO2 did not differ (p > 0.05 for both). For the TT, there was no main effect of HA on total work completed (106.8 ± 23.0 kJ vs. 103.8 ± 21.3 kJ), HR (159 ± 13 bpm vs. 158 ± 13 bpm), SpO2 (83 ± 3% vs. 83 ± 3%), or RPE (17 ± 2 vs. 17 ± 2) (p > 0.05, for all). CONCLUSIONS: Our results suggest that although HA may augment some aspects of ventilation during low-intensity SS exercise, exercise performance measured by a 15 min cycle TT in HH is not affected by HA. Supported by USAMRDC; author views not official US Army or DoD policy