A multilevel meta-analysis of the effects of repeated sprint training in hypoxia on athletic performance.

Background: Repeated-sprint training in hypoxia (RSH) has emerged as a novel strategy to optimize repeated-sprint ability (RSA), aerobic capacity, and anaerobic performance in athletes. Although numerous studies have explored its efficacy compared with repeated-sprint training in normoxia (RSN), inconsistencies remain regarding overall benefits and modulating factors.

Objectives: This study aimed to quantify the overall effect of RSH vs. RSN on athletic performance through a systematic review and multilevel meta-analysis and to identify categorical and continuous moderators influencing intervention efficacy.

Methods: A comprehensive literature search was conducted across PubMed, Embase, Web of Science, and Cochrane Library up to January 31, 2025. Randomized controlled trials comparing RSH and RSN were included. Two reviewers independently screened, extracted, and assessed study quality. Random-effects multilevel models were used to calculate Hedges' g for overall and domain-specific outcomes (RSA, aerobic and anaerobic performance). Categorical (e.g., outcome types, intervention format, sex) and continuous moderators [e.g., inspired oxygen fraction (FiO₂), intervention duration, frequency, and exercise-to-rest ratio] were tested via meta-regression. Publication bias was assessed through funnel plots and regression-based Egger tests.

Results: Eighteen studies (N = 378 participants) were included, yielding 55 independent effect sizes. RSH significantly improved performance outcomes compared with RSN (g = 0.50, 95% CI: 0.34-0.67, p < 0.001). Subgroup analyses revealed stronger effects for RSA (g = 0.61) than for aerobic (g = 0.42) or anaerobic (g = 0.39) outcomes. Moderator analyses indicated that outcome type, exercise format, and FiO₂) significantly moderated the effect size, with lower FiO₂ (-13%-14%) and longer training duration (weeks) associated with greater gains. No sex differences were found. Funnel plot symmetry suggested low risk of publication bias.

Conclusion: This meta-analysis confirms that RSH provides a moderate performance advantage over RSN, particularly for repeated-sprint ability. Specific implementation parameters such as moderate hypoxia intensity and sufficient training duration enhance efficacy. These findings offer evidence-based guidance for optimizing high-intensity interval training protocols under hypoxic conditions.