There has been recent debate on the potential difference in physiological response between exposure to simulated altitude (normobaric hypoxia) and terrestrial altitude (hypobaric hypoxia). Purpose: To determine the difference in the physiological response to normobaric and hypobaric hypoxia during exercise. Methods: Eight recreationally active subjects (27 ¡ 5 years old, 73.1 ¡ 7.4 kg body weight, 170.6 ¡ 6.7 cm height, and 19.3 ¡ 9.2% body fat) completed incremental cycling exercise to volitional fatigue in three separate environments: normobaric normoxia (NN; 350 m), normobaric hypoxia (NH; simulated 3094 m), and hypobaric hypoxia (HH; 3094 m). Heart rate, blood oxygen saturation, and muscle tissue oxygenation were measured at rest and continuously throughout the exercise trials. Results: Blood oxygen saturation (SpO2) was ,10% higher in NN compared to the two hypoxic conditions (p , 0.001) at rest and all exercise stages, with no difference between NH and HH (p . 0.05). Heart rate was higher at rest in HH (98 ¡ 13 bpm) compared to NN (83 ¡ 15 bpm, p 5 0.011) and NH (84 ¡ 14 bpm, p 5 0.001) which persisted until 165 watts at which point no difference was observed (p . 0.05). Muscle tissue oxygenation was 17% higher in HH compared to NN and 19% higher than NH throughout exposure (p , 0.05). Conclusion: These data indicate that the hypoxic stresses resulting from normobaric and hypobaric hypoxia are not the same and that hypobaric hypoxia may not result in hypoxia at the level of the tissue.

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There has been recent debate on the potential difference in physiological response between exposure to simulated altitude (normobaric hypoxia) and terrestrial altitude (hypobaric hypoxia). Purpose: To determine the difference in the physiological response to normobaric and hypobaric hypoxia during exercise. Methods: Eight recreationally active subjects (27 ¡ 5 years old, 73.1 ¡ 7.4 kg body weight, 170.6 ¡ 6.7 cm height, and 19.3 ¡ 9.2% body fat) completed incremental cycling exercise to volitional fatigue in three separate environments: normobaric normoxia (NN; 350 m), normobaric hypoxia (NH; simulated 3094 m), and hypobaric hypoxia (HH; 3094 m). Heart rate, blood oxygen saturation, and muscle tissue oxygenation were measured at rest and continuously throughout the exercise trials. Results: Blood oxygen saturation (SpO2) was ,10% higher in NN compared to the two hypoxic conditions (p , 0.001) at rest and all exercise stages, with no difference between NH and HH (p. 0.05). Heart rate was higher at rest in HH (98 ¡ 13 bpm) compared to NN (83 ¡ 15 bpm, p 5 0.011) and NH (84 ¡ 14 bpm, p 5 0.001) which persisted until 165 watts at which point no difference was observed (p. 0.05). Muscle tissue oxygenation was 17% higher in HH compared to NN and 19% higher than NH throughout exposure (p , 0.05). Conclusion: These data indicate that the hypoxic stresses resulting from normobaric and hypobaric hypoxia are not the same and that hypobaric hypoxia may not result in hypoxia at the level of the tissue.