There are few greater powerhouses of distance running than Ethiopia. In the thin air of the forests above Addis Ababa, you’ll find the world’s greatest going about their business, day after day, week after week, month after month, in pursuit of lactate loving glory. The high altitude has always been a part of the Ethiopian mystique; essential but not sufficient on it’s own for distance dominance. Down below, at the relatively modest elevation of 1800 m above sea level, lies the Bahir Dar University, home of research into the effects of altitude training on distance running, from which a new meta analysis has been published in Physiological Reports, outlining the extent to which altitude training may benefit performance metrics, and crucially unpicking the potential mechanisms and best protocols for our own training.
Meta Analysis
But first, what is a meta analysis, and why should we care about it’s outcome? Scientific research is built upon individual research papers. A group of scientists have an idea, and they run an experiment on maybe 10-20 people if we’re lucky, and then they write about it; what they did, what they found out, what questions still remain. This is the keystone of research, and remains absolutely essential, however individual studies may be too small to draw wider inference, or to detect statistically signifcant impacts. By pooling the data from every participant in every study, researchers can draw much stronger conclusions about the size of the effect a certain intervention (in this case altitude training) can have. It also allows researchers to delve further into subgroups (for example differentiating by sex, fitness level etc) to find out where the intervention is best applied, and how. In short, provided the studies included in the meta analysis are sound, the results from a meta-analysis can be some of the strongest scientific evidence we have to answer a question.
The Current Research
The authors of this meta analysis had two key questions they wanted to answer:
- Does the pooled effect indicate a significant VO2 max improvement in the combined HIIT and hypoxic conditions? (I.e. does high intensity interval training at altitude improve VO2max to a greater extent than training at sea level.
- How do subgroup differences such as hypoxic type, training status, training week, and sex influence theVO2 max of middle- and long-distance athletes? (I.e. who may benefit the most and how should we use altitude)
What They Did
To answer these questions, the authors took the raw data from 6 studies and pooled the data, allowing them to run new analyses to answer the above questions. This may not seem like lots of papers, but the authors were very strict on the quality of the paper and their inclusion critera so as to reduce risk of bias and low methodological quality. For reference, that reduced the number of eligible papers from 1183 to the final 6! In total, it yielded a total sample size of 116 runners.
What They Found
The headline from the paper is undoubtedly the finding that interval training in hypoxic conditions improves VO2max to a greater extent than training at sea level. Individual studies included in the meta-analysis reported VO2 max improvements ranging from 4.4% to 13.6% in hypoxic training groups, while normoxic training groups showed increases from 1% to 8.3%, and in some cases, reductions. The authors argued that the additional physiological stress of hypoxia leads to the improvements in VO2max, by stressing the system’s ability to take in, supply and use oxygen.
Subgroup analyses did not reveal any major differences when considering the type of hypoxia (hypobaric vs. normobaric), training status (trained vs. highly trained), intervention week (duration of the intervention), and sex. That means these factors were not significantly related to the overall pooled effect size on VO2 max improvement in middle- and long-distance athletes, however the authors did note that it is possible these sub groups were still slightly underpowered to detect small effects.
Of note is the fact that the studies included in the meta-analysis primarily used “long HIIT” protocols with interval bout durations ranging from 5 to 30 minutes and work intensities between 90% and 95% of maximum heart rate (HRmax). Recovery periods varied. This is one type of HIIT training available, however it is probably the most commonly used by middle and long distance runners considering typical track type workouts and interval training that might take place on road.
Load Management
There is an interesting phenomena with training at altitude in which the internal and external workload become dissociated from one another. That is, if running at a set speed at sea level, you would expect to see a certain heart rate (e.g. 15 kph = 160 bpm). However at altitude, for the same speed, you would expect a higher heart rate (i.e. 15 kph now = 170 bpm at altitude). This means that to run at the same speed at altitude forces a greater physiological stress, to which the authors attributed some of the performance improvement. However, we may flip this on this its head, and say that we will hold heart rate consistent across altitude and sea level, and accept that we will run at a lower speed at altitude. Interestingly, even in studies which account for this and allow a lower running speed at altitude, we still see these improvements. In fact, one study even showed increased oxidative capacity in athletes who trained at a lower internal workload (i.e. lactate level) at altitude compared to those who trained at a higher internal workload at altitude. This shows that we can, and should, afford a slight drop in absolute intensity at altitude to still realise the benefits of altitude training, without overstretching ourselves and risking burnout or injury.
The Oxygen Question
The current research is interesting in its finding, as some practitioners and researchers have recently pushed back against performing HIIT at altitude. It is generally considered that the most important factor in a session’s efficacy in improving VO2max, is time spent at or near to VO2max (that is, the maximum volume of oxygen you can take in and use per minute). In order to stress this, we therefore require high oxygen flux through the system. By limiting environmental oxygen availability at altitude, there is a risk that oxygen flux is reduced, and therefore it is not possible to achieve the same VO2 at altitude as you would in a session at sea level. While this does seem to be the case, the present research suggests that this is acceptable, because although the absolute VO2 is reduced, so is your VO2max at high altitude, therefore the relative percentage of VO2max at which you are working is maintained. This question probably requires further investigation to establish a consensus.
Limitations
As with any study, this meta analysis is only one piece of the puzzle, and is not without limitations. Chief amongst these remains sample size. While 116 participants is still more than any individual study could hope to recruit, it has limited the authors’ ability to infer results about speciifc sub groups. This is especially compounded by under-representation of women amongst participants in many of these studies which limits our ability to infer sex differences. Further, although the overall results of the meta-analysis are fairly clear, one included study found no difference between a sea level group and altitude counterparts- understanding why this study did not observe differences is just as important as understanding that the majority did, so that we can tailor interventions to avoid these mistakes in future.
Take Home
The key take home message is that regardless of who you are, you stand to gain from performing interval training in hypoxic conditions. Longer intervals, as performed by middle and long distance type runners, are especially useful in brinigng about these physiological and performance improvements
Fentaw, S., Tadesse, T., & Birhanu, Z. (2025). Methodological and aerobic capacity adaptations of high-intensity interval training at different altitudes in distance runners: A comprehensive meta-analysis. Physiological reports, 13(9), e70349. https://doi.org/10.14814/phy2.70349