Acute and post-exercise physiological responses to different hypoxic exercises

A better understanding of the acute and post-exercise physiological responses to hypoxic exercise (i.e. exercise combined with hypoxic stress) can provide knowledge to improve human tolerance to high altitude environments, also providing helpful information for the adoption of safe exercise protocols in individuals engaged in hypoxic exercise training. This doctoral thesis investigated the acute exercise and post-exercise physiological responses evoked by hypoxic exercises of various intensities and nature, providing new insights into the understanding of the complex interaction between exercise and hypoxic stimuli in inducing the aforementioned responses. Study 1 investigates the effects of hypoxia (FiO2=13.4%, ≈3500 m) on the acute exercise and post-exercise cardiorespiratory and cardiac autonomic responses to a maximal cardiopulmonary exercise test. The study shows that cardiac autonomic recovery is delayed in response to a maximal cardiopulmonary exercise test in hypoxia and that the degree of cardiac autonomic recovery impairment is directly related to the increase of exercise-induced physiological stress associated with hypoxic exercise. Study 2 aims at examining the effects of hypoxic exercise (FiO2=14.2%, ≈3000 m) performed at the same absolute intensity (i.e. 80% of the power output at the first ventilatory threshold) or same relative intensity (i.e. heart rate matched exercise) of normoxic exercise on the exercise cardiac autonomic and cardiorespiratory responses. The study shows that moderate heart rate matched hypoxic exercise triggers similar cardiac autonomic and physiological responses to normoxic exercise with a reduced mechanical load; whilst the same absolute intensity exercise in hypoxia is associated with increased exercise-induced physiological stress and delayed cardiac autonomic recovery. Study 3 aims at examining the effects of hypoxic exercise (FiO2=14.2%, ≈3000 m) performed at the same absolute (i.e. work rate matched exercise) or same relative (i.e. heart rate matched exercise) normoxic exercise intensity on the post-exercise cardiac autonomic and cardiovascular responses. The study shows that moderate heart rate matched hypoxic exercise (≈75% HRmax) does not affect cardiac baroreflex sensitivity and does not blunt cardiac autonomic recovery during post-exercise recovery, but does not induce significant post-exercise hypotension. Conversely, work rate matched hypoxic exercise, resulting in greater physiological stress, delays cardiac autonomic recovery, temporarily decreases cardiac baroreflex sensitivity and evokes prolonged post-exercise hypotension. Study 4 aims at investigating the effects of a simulated mountain hike in a cold and hypoxic environment (-25°C, FiO2=11%, ≈5000 m) on cardiac autonomic activity, and the influence of two different strategies (i.e. different work-rest durations, long vs short) on the physiological and perceptual responses associated with the simulated ascent. The study shows that reduced work-rest durations are associated with improved perceptual responses and less perturbation of cardiac autonomic balance compared to longer work-rest durations in response to a simulated hike at high altitude. Investigating the acute exercise and post-exercise physiological responses evoked by hypoxic exercises of various intensities and nature, this doctoral thesis and the data presented herein want to expand our understanding of hypoxic exercise and stimulate new research on this topic.