What is the process of adapting the heat, and how much can it help improve your tolerance and performance in hot weather? It does not take a meteorologist to know that summers seem to be getting hotter every year. It also doesn’t take multiple academic degrees to realize that it’s much more challenging to ride hard or long when the temperatures heat up.
Mark Cavendish suffered on Tour’24 stage 1
Heat and Performance
The first systematic studies on heat stress and performance were performed on US soldiers during the Second World War and clearly showed reduced physical capacity. At my postdoctoral lab in Scotland, Galloway & Maughan (1997) had subjects exercise on cycle ergometers at four different temperatures to voluntary exhaustion: 4, 11, 21, and 31°C. They observed the longest tolerance time at 11°C, with progressively worse performance at 21 and then 31°C.
This relatively cool temperature for optimal performance was corroborated by Ely et al. (2007) analysis of marathon times across many events in different weather conditions. These authors also found that fitter runners were relatively less impaired and less fit and slower runners. In turn, this supports my own PhD work that found that fitter individuals with a higher aerobic capacity could tolerate exercise in the heat longer and also higher core temperatures before volitional exhaustion.
Heat Adaptation
So in general, heat is bad but physical fitness offers some protection. However, the biggest form of protection appears to be through a process of gradual adaptation to heat, brought about by physiological changes in the body. My UK colleague Chris Tyler prepared a large meta-analysis review on all existing research (96 papers) studying heat adaptation and exercise performance in 2016 (Tyler et al. 2016).
Since 2016, and partly thanks to many more major sporting events taking place in very hot locations (UCI Worlds 2016 in Qatar, World Athletics Championships 2019 in Qatar, Tokyo Olympic Games in 2021, FIFA World Cup 2022 in Qatar), so much more research has occurred in heat adaptation that it was time for Chris and me to “get the band back together again” and update the meta-analysis with an additional 39 new papers (Tyler et al. 2024).
As a brief primer, a meta-analysis is a systematic analysis of all of the existing literature on a particular topic. Having collected all the relevant papers, a meta-analysis then takes all of the findings from these papers and quantifies them to allow an overall analysis.
In our case, we were primarily focused on whether heat adaptation actually improves athletic performance and also by how much. Along with this, we tried to quantify the dosage required to become adapted: the length of adaptation (five days or less, two weeks or less, or greater than two weeks), the mode of adaptation (passive exposure as in the sauna, exercise in neutral or hot environments, etc.), differences between men and women, and the effects of fitness.
Primary Findings
Sifting through all the studies certainly made us a little bit cross-eyed and also killed our own heat adaptation, what with all the time in front of the computer screen indoors!
Overall, our 2024 update largely corroborated our 2016 review. What were some of our key findings?
• Heat adaptation clearly improves both exercise tolerance and also performance capacity. The majority of studies that we analysed reported an improved performance, with relatively few demonstrating no significant changes and none reporting that a process of heat adaptation negatively affected performance. Overall, the improvement in performance was rated as moderate but clearly significant.
• Most of the protocols were in the range of 7 to 14 days, with a fair amount that were in the 4-5 day range and only a few that were longer than 14 days. This was not surprising given the logistical challenges of prolonged adaptation studies either in the field or in the lab.
• The good news is that even short-term adaptation protocols or 4-5 days were generally effective in eliciting improved performance. The longer the protocol duration, the greater the magnitude of improvement. So if at all possible, the longer you can pre-adapt to heat the better. If you have to, this may require re-creating what Bradley Wiggins did before the Vuelta 2011, famously training indoors in his backyard greenhouse with some heaters in preparation.
• Overall, the key stimulus that drives heat adaptation is a sufficiently high level of exercise and/or heat to drive up core temperature for a sustained period of time each session. One thing to keep in mind is that, as the body physiologically adapts to being able to handle greater heat loads, the intensity of exercise and/or the heat load must also progressively increase to maintain this thermal stimulus.
• Another important thing to remember is that different physiological responses each have their own timeline for adaptation. Plasma volume rapidly increases in the early stages of adaptation, with decreasing heart rate and core temperature (both at rest and during steady-state exercise) adapting next. The slowest response is an increased rate of sweating, which commonly requires 14 or more days for maximal response.
• Not too many studies investigated the effects of heat adaptation on perceptual response and tolerance, but those that did generally observed an improved subjective tolerance to heat. Given recent findings over the past two decades on the strong linkages between psychological perception and subsequent willingness to exercise, this perceptual adaptation to heat may be equally as important as any physiological adaptation.
• Only a very few studies looked at whether age or aerobic capacity had any influence on the benefits of heat adaptation to performance. It is difficult to generalize from such limited data as a result. However, one thing to keep in mind is that fitter individuals will need to have much harder exercise levels in order to sufficiently stress the body thermally and, therefore, the exercise regimen will likely need to be individually designed for each athlete.
Summary
Of all the potential countermeasures for exercising in that heat, a prolonged period of pre-adaptation should really serve as the bedrock foundation. That is because, compared to acute interventions such as pre-cooling and hydration, heat adaptation is spread across multiple physiological systems and is also well sustained for a prolonged period of time.
Two key things to keep in mind throughout this adaptation:
1. Expect some impairment in exercise capacity during acute exposure to heat or the early days of the heat adaptation protocol, so do not plan to jump right into heavy intervals or racing right away. Some research even suggests that heat training negatively impacts your body’s physiological adaptation to training.
2. On the flipside, gradually ramp up your training load over the adaptation period, as this is critical to maximize the thermal stimulus that you are getting.
Have a cool hot summer!
Keep cool
Further Resources
We’ve published a lot of articles about heat here on Pez. Below are a selection:
Can heat training work for cool weather performance?
Can heat training slow you down?
Can you heat adapt by wearing lots of clothing indoors?
Mental training for hot weather performance
I’ve also done a lot of podcast interviews about heat and heat adaptation. Check out some recent ones in Canadian Cycling Magazine Podcast and the Consummate Athlete Podcast.
References
Ely MR, Cheuvront SN, Roberts WO & Montain SJ. (2007). Impact of weather on marathon-running performance Med Sci Sports Exerc 39, 487-493.
Galloway SD & Maughan RJ. (1997). Effects of ambient temperature on the capacity to perform prolonged cycle exercise in man Med Sci Sports Exerc 29, 1240-1249.
Tyler CJ, Reeve T, Hodges GJ, & Cheung SS. (2016) The effects of heat adaptation on physiology, perception and exercise performance in the heat: a meta-analysis Sports Med. 46(11):1699-1724. https://www.ncbi.nlm.nih.gov/pubmed/27106556
Tyler CJ, Reeve T, Sieh N, & Cheung SS. (2024) Effects of Heat Adaptation on Physiology, Perception, and Exercise Performance in the Heat: An Updated Meta-Analysis. J Science Sport Exerc. https://doi.org/10.1007/s42978-023-00263-8
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