2.5. Breath holding and steady‐state hypoxia

Three separate breath holds were performed following the HVR protocol. Participants were first coached through two, randomized, maximal breath‐hold maneuvers initiated at the end of a normal inspiration: one following breathing room air (normoxia), and another following breathing five normal tidal breaths of 100% F_IO₂ from a 50 L Douglas bag (i.e., hyperoxia). For these two randomized maneuvers, breath holds were initiated when all variables returned to baseline breathing room air (~5–10 min). Following a 10‐min break, the participant was then exposed to a F_IO₂ of approximately 0.135–0.14 (13.5%–14%), equating to simulated 4,500–5,000m of altitude. Once the participant reached steady state (i.e., unchanging variables; e.g., ventilation, P _ETCO₂, and SpO₂; ~30 min), they performed the third breath‐hold maneuver. The initiation of each breath hold began with five coached breaths at their normal (i.e., resting) tidal volume to avoid anticipatory changes in breathing rate or tidal volume (leading to hypocapnia), and each breath hold was held until volitional break point. No practice breath holds were performed, nor were participants given encouragement throughout the breath hold. Each participant completed testing during a single laboratory visit (~2 hr).