This study will consist of a parallel-groups design, with 30 healthy active female participants randomly assigned to either an experimental heat acclimation and exercise intervention, or a thermo-neutral exercise intervention control group. Interventions will be 10 days in duration, and consist of daily 60-minute exercise bouts under the appropriate environmental condition. Before and after each intervention, various tests will be conducted to establish exercise capacity under various environmental conditions, as well as underlying mechanisms of physiological adaptation induced by each intervention.

Most research on how the human body responds to different environmental conditions has primarily focused on men, leaving a gap in our understanding of how women adapt to these conditions. Heat exposure in particular is known to affect the well-being and performance of humans, as well as induce chronic adaptations through an acclimation/acclimatization process, which helps the body to better regulate core temperature. Moreover, contemporary research is beginning to explore the 'cross-tolerance' phenomenon; the notion that exposure (and acclimation/acclimatization) to one environmental stressor may affect the responses to another. In particular, both heat and hypoxia are known to activate common acclimatization pathways, with pulmonary, cardiovascular, hematological and muscular adaptations occurring to facilitate both oxygen transport and core body temperature regulation. In line with this background, the primary aim of this study is to investigate the effects of a heat acclimation and exercise intervention, relative to a thermo-neutral exercise control intervention, on exercise tolerance under various environmental conditions (heat, hypoxia, neutral) in healthy, active women. The secondary aim is to establish mechanisms of adaptation, by exploring the intervention-induced changes in pulmonary, cardiovascular, hematological and muscular factors, through various tests conducted at rest and during exercise.

To address these aims, 30 healthy active female participants, aged between 18 and 35 years, will be randomized to either an experimental (heat acclimation and exercise training) or control (thermo-neutral exercise training) group. The experimental group will complete a 10-day heat acclimation training intervention, exercising for 60 minutes per day in a climactic chamber set to 35°C and 50% relative humidity. The control group will complete a similar exercise intervention, but under thermo-neutral conditions (23°C and 50% relative humidity). Before and after the intervention period, both groups will complete a wide variety of tests, including exercise capacity measurements under each environmental condition (heat [35°C], hypoxia [Fraction of inspired oxygen = 0.135], neutral [23°C, FiO2 = 0.209]), body composition assessments, lung function testing, hemoglobin mass and blood volume quantification, skeletal muscle mitochondrial function and vascular responsiveness assessment, and venous blood sampling for a variety of sex hormone-, redox balance-, hematopoietic-, heat shock protein-, hypoxia-inducible factor- and genetic-related markers.

Inclusion Criteria:

• Age: 18 - 35 years
• Body mass index: 18.5 - 25.0 kg/m^2
• Regular physical activity (at least 30 minutes of moderate-intensity activity, three times per week)

Exclusion Criteria:

• Habitual smoker within the past 5 years
• History of metabolic disorders or any medications deemed to pose an undue risk or introduce bias in any outcome measures
• Exposure to altitude > 2000 m above sea level within four weeks of scheduled participation
• Permanent residence at altitude > 1000 m above sea level

Participants will be recruited based on their self-identification as female. Sex hormone measurements will later be conducted to verify alignment between their gender identity and biological sex.

All of the individual participant data collected during the study, after de-identification, will be made publicly available using an online data repository.

Data will be made available immediately following publication with no end date.

Any purpose.

Considering that the heat acclimation condition will be easily perceptible to the participants, it will not be possible to blind them from their group assignment. The investigators will also be required to set the environmental conditions for each exercise session, so they will also be aware of group assignment during the intervention. However, efforts will be made to blind outcomes assessors to group assignment, by reassigning unique participant identifier codes with the random assignment of "A" and "B" to the experimental and control groups by a third party. The experimental and control group can therefore be compared, without knowledge of which label corresponds to which group until after data processing and statistical analyses have been performed.