Hypoxic exposure and air-quality reporting in normobaric hypoxic resistance training: a systematic evidence map and exploratory meta-analysis

Normobaric hypoxic resistance training (NHRT) studies describe the hypoxic prescription using the fraction of inspired oxygen (FiO₂) or simulated altitude. However, FiO₂ is an external setting and may not fully represent the hypoxic stimulus experienced during training. Training SpO₂ responses, the hypoxia-delivery method, facility volume, gas flow, ventilation, CO₂, humidity and temperature may affect the exposure environment. These variables are important for interpreting and comparing NHRT studies, but their reporting has not been systematically evaluated.

This evidence map assessed reporting of hypoxic exposure and air quality in chronic NHRT studies and conducted exploratory meta-analyses when outcomes and data were comparable. The primary evidence map comprised 16 independent chronic NHRT cohorts. Reporting completeness was assessed in this set and repeated in an extended 17-study set that included one additional air-quality reference study. Thirteen independent primary cohorts contributed to at least one exploratory meta-analysis. NHRT-versus-NRT syntheses used random-effects models and Hedges-corrected standardised mean differences.

FiO₂ or simulated altitude was reported relatively well, whereas training SpO₂, CO₂, humidity, temperature, ventilation, space and flow-related variables were underreported. Thirteen independent cohorts contributed to at least one exploratory meta-analysis. Lower-limb maximal strength (7 studies; SMD = 0.07, 95% CI -0.26 to 0.41; I² = 0%) and lean mass (5 studies; SMD = 0.12, 95% CI -0.25 to 0.49; I² = 0%) did not show a clear additional advantage of NHRT over NRT. Whole-body body fat percentage showed an unstable numerical direction favouring NHRT (5 studies; SMD = -0.33, 95% CI -0.78 to 0.12; I² = 23.2%). Local muscle morphology and local muscular endurance outcomes were supported by few studies and should be interpreted as exploratory.

Existing NHRT studies report FiO₂ or simulated altitude relatively well, but actual exposure conditions and internal hypoxic responses remain incompletely described. Exploratory meta-analyses did not show that NHRT was superior to NRT for common outcomes related to strength, body composition or local muscle morphology. Future NHRT studies should report not only FiO₂, but also hypoxia-delivery method, internal oxygenation responses and air-quality control conditions.

PROSPERO CRD420261405733.