Bonetti, D. L., & Hopkins, W. G. (2009). Sea-level exercise performance following adaptation to hypoxia: A meta-analysis. Sports Medicine, 39, 107-127.

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This report is the first meta-analytic review of the effects on performance and related physiological measures following adaptation to six protocols of natural or artificial hypoxia: live-high train-high (LHTH), live-high train-low (LHTL), artificial LHTL with long continuous daily exposure (8-18 hours), brief (1.5-5 hours) continuous or brief (<1.5 hours) intermittent periods of hypoxia, and artificial live-low train-high (LLTH). Qualifying studies (N = 51) provided 11-33 estimates for effects on power output [a general physical factor rather than specific performance indicator] with each protocol and up to 20 estimates for effects on maximal oxygen uptake (VO2max) and other potential mediators. Poor reporting of inferential statistics [and study designs and controls] limited the weighting factor in the models to sample size. Probabilistic inferences were derived using a smallest worthwhile effect on performance of 1%.

Substantial enhancement of maximal endurance power output in controlled studies of subelite athletes was very likely with artificial brief intermittent live-high train-high (~2.6%), likely with live-high train-low (~4.2%), possible with artificial long continuous live-high train-low (~1.4%), but unclear with live-high train-high (~0.9%), artificial brief continuous live-high train-low (~0.7%), and live-low train-high (~0.9%). With elite athletes, enhancement was possible with natural live-high train-low (~4.0%), but equivocal with other protocols. There was evidence that these effects were mediated at least partly by substantial placebo, nocebo, and training-camp effects with some protocols. [The influence of psychological factors (e.g., expected effects) is a common weakness with altitude studies.] Enhancing protocols by appropriate manipulation of study characteristics produced clear effects with all protocols (~3.5-6.8%) in sub-elite athletes, but only with live-high train-high (~5.2%) and live-high train-low (~4.3%) in elite athletes [although other uncontrolled factors could have produced these results]. For (VO2max), increases were very likely with live-high train-high (~4.3%) in sub-elite athletes, whereas in elite athletes a 'reduction' was possible with live-high train-high (~-1.5%); changes with other protocols were unclear. [Note that VO2max was increased with sub-elite athletes but reduced with elite athletes.] Effects on erythropoietic and other physiological mediators provided little additional insight into mechanisms.

Implication. Natural live-high train-low currently provides the best protocol for enhancing endurance performance in elite and sub-elite athletes, although the effects could be due to psychological and mediating environmental factors. Some artificial protocols are effective in sub-elite athletes. Likely mediators include (VO2max) and the placebo, nocebo, and training-camp effects.

This study did not necessarily look at performance, but rather a clinical performance indicator (power output). Most research looks at physiological "mediators", which are not necessarily indicative of specific performance capabilities. As well, the authors indicated that reported effects are influenced by psychological factors (e.g., placebo, nocebo). [Where appropriate, most studies do not control for equal work or living conditions in experimental and control conditions.]

The conclusions in this area of investigation are not reliable until the experimental designs and controls are better structured and observed. With confounding variables and lack of controlled conditions between the "high" and "low" environments, no conclusive relationship suggesting performance benefits from any variant of altitude training are evident in elite athletes. Benefits to sub-elite athletes are possible but most likely due to psychological effects and changes in environmental conditions.

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