The Effects of Intermittent Hypoxic Training Strategies on Maximal Oxygen Uptake in Healthy Humans: A Meta-analysis with Meta-regression.
BACKGROUND: Intermittent hypoxic training aims to enhance exercise performance and health, commonly assessed through maximal oxygen uptake (VO2max). Currently applied methods are live-high train-low (LHTL), live-low train-high (LLTH) and passive hypoxic conditioning (PHC). However, within these methods, numerous modes of application exist, and effectiveness is debated. OBJECTIVE: To determine the effect of LHTL, LLTH and PHC on VO2max in healthy participants, and identify factors associated with the amplitude of change in VO2max. METHODS: We conducted a systematic review with random-effects meta-analysis and meta-regression. Studies were identified through PubMed, EMBASE and Web of Science. We included controlled studies, calculating between-group standardised mean differences. Athlete and non-athlete populations were analysed separately. RoB2 was used to assess study quality and publication bias was investigated with funnel plots and the Egger's test. RESULTS: Of the 5244 identified studies, 62 were included for analysis, resulting in a sample size of 1332 participants (610 athletes, 722 non-athletes). LHTL increased VO2max significantly more than control in athletes (mean difference [MD] = 2.17; 95% CI = 0.77, 3.56) but not in non-athletes (MD = 3.1; 95% CI =  - 0.23, 6.43). LLTH did not increase VO2max significantly more than control in athletes (MD = 0.89; 95% CI =  - 0.27, 2.05) but it did in non-athletes (MD = 1.70; 95% CI = 0.85, 2.54). PHC did not increase VO2max significantly more than control in athletes (MD =  - 1.07; 95% CI =  - 3.51, 1.37) but it did in non-athletes (MD = 2.26; 95% CI = 1.00, 3.52). Multivariate meta-regressions identified the severity, duration, and frequency of hypoxic exposure to be significantly associated with the change in VO2max. CONCLUSIONS: LHTL showed a significant effect on VO2max in athletic populations while LLTH and PHC showed a significant effect on VO2max in non-athletic populations only. Future studies should target investigating which characteristics (e.g. severity, duration) of the hypoxic exposure are most beneficial.