Heat Training as a Performance Enhancer
Heat training is becoming increasingly popular because of the benefits associated with it. It is well understood that if you are competing in a hot environment, you should expose yourself to those conditions beforehand. Usually, athletes preparing for competitions in these hot environments spend days or weeks training in the heat to reduce the risk of heat illness and improve comfort during competition.
There is a lot of evidence out there showing how heat acclimation may be one of the most powerful performance interventions available. Research over the last 15 years has demonstrated that repeated heat exposure can improve endurance performance, increase plasma volume, enhance cardiovascular function, and improve lactate threshold. The cool thing is that these adaptations can occur surprisingly quickly, often within five to ten sessions.
This article summarizes findings from several heat acclimation papers and explores how coaches and athletes can apply heat training to improve performance.
Preparing for Competition in the Heat
More people are becoming aware of the physiological benefits of heat acclimation. Repeated exposure to hot environments produces adaptations that improve the body's ability to dissipate heat, maintain cardiovascular stability, and reduce physiological strain. Some other adaptations include:
Increased plasma volume
Earlier onset of sweating
Increased sweat rate
Reduced sweat sodium losses
Lower heart rate during exercise
Lower core temperature
Improved skin blood flow
Improved fluid balance
Increased cellular protection through heat shock proteins
Together, these adaptations allow athletes to tolerate heat stress more effectively while maintaining performance.
However, researchers eventually began asking a different question: Can we train in the heat in order to improve performance, even if the competition is not held in the heat?
Study: Using Heat Training for Improved Performance
In 2010, there was a study where twelve trained cyclists completed a 10-day heat acclimation intervention consisting of 90 minutes of cycling each day in a 40°C (104°F) environment. A matched control group performed identical training in cool conditions (Lorenzo et al., 2010).
After 10 days, the heat-acclimated athletes were tested in cool and hot conditions, and the results were:
5% increase in VO₂max in cool conditions
8% increase in VO₂max in hot conditions
5% increase in power at lactate threshold
6% improvement in time trial performance in cool conditions
8% improvement in time trial performance in hot conditions
The control group, who did not do the training in the 40°C environment, experienced no meaningful changes.
The cool thing that this study showed was that heat-trained athletes improved performance even when tested in cool conditions. This suggests that heat acclimation can be an ergogenic aid capable of enhancing endurance performance more broadly. The researchers attributed many of these benefits to plasma volume expansion, which increased by approximately 6.5%.
How Quickly Do Adaptations Occur?
The amount of time required to complete a heat acclimation block is not as much as you’d think. There are several studies which suggest that meaningful adaptations occur much faster than previously believed.
It has been shown that as little as five days of heat acclimation can improve physiological determinants of endurance performance and can enhance 5-km running performance (Garrett et al., 2012). A systematic review by Chalmers and colleagues concluded that four to seven heat exposures are often sufficient to induce substantial aerobic performance benefits (Chalmers et al., 2014).
From a practical standpoint, it’s been shown that athletes do not necessarily need two weeks of dedicated heat training to benefit. Improvements can occur within a single week.
How Long Do Adaptations Last?
Just like everything else with fitness and performance, the heat adaptations are not permanent. There is evidence suggesting that physiological adaptations begin to decay once heat exposure ceases. Although some benefits may persist for several weeks, meaningful reductions can occur within days (Garrett et al., 2011).
Even though the gains are lost quickly, the reacclimation phase occurs much faster than initial acclimation. Athletes who have previously completed a heat acclimation block can regain adaptations after only a few days of renewed exposure. This means you can strategically maintain adaptations with periodic heat sessions throughout a competitive season.
Why Does Heat Training Work?
As we’ve mentioned above, the benefits of heat training are primarily attributed to plasma volume expansion. So the question becomes, what is increased plasma volume/plasma volume expansion? Why is it good?
When plasma volume increases, there are several beneficial adaptations that follow: increased venous return, increased stroke volume, greater cardiac output, reduced cardiovascular strain, and improved thermoregulation. Essentially, the heart becomes more efficient at delivering oxygen and removing heat.
Heat-acclimated athletes maintain lower heart rates and a lower core temperature at the same workload, which means a greater proportion of physiological resources can be used for performance rather than cooling the body.
Physiology Side Note: What is increased plasma volume? Why is that good?
Blood consists of plasma (~55%) and blood cells (~45%). Plasma is the liquid portion that contains electrolytes, proteins (albumin, globulins), hormones, nutrients, and waste products. When these research papers say that heat acclimation increases plasma volume by 5–15%, they mean you have more fluid circulating through your vascular system.
How does the body increase plasma volume?
Repeated heat exposure causes heavy sweating, temporary fluid loss, and an increased secretion of Aldosterone and Vasopressin (ADH), which are hormones that cause the kidneys to retain water and sodium, which in turn pulls water into the blood stream. This ultimately causes more fluid inside the blood vessels.
Cardiovascular effects
This increase in blood volume leads to a cascade effect. Blood volume is higher, which leads to more venous return, greater ventricular filling, higher stroke volume, and thus a lower heart rate at the same workload. After heat training, your heart rate will be lower and your stroke volume will be higher, at a specific workload. Further, VO₂max increases because of this increased stroke volume and higher maximal cardiac output.
Thermoregulation
When you are exercising, your body must deliver blood to working muscles and the skin (especially if you are training in the heat). If there is not sufficient blood volume, these systems compete. With increased blood volume, these systems compete less, so you can maintain muscle blood flow and maintain skin blood flow, which reduces heart rate and perceived exertion.
So…How can you Heat Train?
There are several ways you can heat train, depending on what is available to you. Some athletes have great resources and access to environmental chambers, but that’s not an easy thing to find. Some athletes live in hot climates which makes heat training accessible, but many coaches are reluctant to compromise training quality by conducting key sessions in hot conditions. One way to utilize the hot climates is to do easy training sessions in the heat, versus doing quality sessions in the heat.
In one study, it was shown that six consecutive days of post-exercise hot water immersion improved heat adaptation and enhanced 5-km running performance in hot conditions. Participants completed normal training and then sat in 40°C water for approximately 40 minutes after exercise (Zurawlew et al., 2016). Other studies have shown sauna-based interventions have demonstrated increases in plasma volume and endurance capacity. These are passive methods.
Athletes can obtain many of the benefits of heat acclimation without substantially modifying their primary training sessions.
Practical Takeaways
Passive exposure methods:
Consider incorporating post-exercise sauna sessions 2–4 times per week.
Begin with 15–20 minutes and gradually progress.
Maintain adequate hydration.
Use heat exposure as a supplement to training, not a replacement.
If you have a competition:
A 5–10 day heat acclimation block can produce meaningful adaptations.
Aim for 60–90 minutes of heat exposure daily.
Target core temperatures above approximately 38.5°C.
If you do not have ways to measure core temperatures, try measuring ear temperature as a proxy.
Schedule acclimation 1–3 weeks before competition.
Use maintenance exposures if competition is delayed.
What Is the Overall Message?
In the references below, you will find over ten papers regarding heat training. The overall message is that heat acclimation is one of the most effective physiological interventions available for endurance performance.
Detailed throughout this article, there are many benefits that come up as a result of heat training. Heat acclimation is much more than preparation for hot-weather competition; it can be utilized as a legitimate performance-enhancement strategy.
I believe many coaches and athletes know that heat acclimation works. The challenge is determining how to integrate it into a training plan in a way that maximizes adaptation while preserving training quality.
References
1. Garrett, A. T., Rehrer, N. J., & Patterson, M. J. (2011). Induction and decay of short-term heat acclimation in moderately and highly trained athletes. Sports Medicine, 41(9), 757–771. https://doi.org/10.2165/11587320-000000000-00000
2. Pryor, J. L., Johnson, E. C., Roberts, W. O., & Pryor, R. R. (2019). Application of evidence-based recommendations for heat acclimation: Individual and team sport perspectives. Temperature, 6(1), 37–49. https://doi.org/10.1080/23328940.2018.1516537
3. Lorenzo, S., Halliwill, J. R., Sawka, M. N., & Minson, C. T. (2010). Heat acclimation improves exercise performance. Journal of Applied Physiology, 109(4), 1140–1147. https://doi.org/10.1152/japplphysiol.00495.2010
4. Chalmers, S., Esterman, A., Eston, R., Bowering, K. J., & Norton, K. (2014). Short-term heat acclimation training improves physical performance: A systematic review, and exploration of physiological adaptations and application for team sports. Sports Medicine, 44(7), 971–988. https://doi.org/10.1007/s40279-014-0178-6
5. Casadio, J. R., Kilding, A. E., Cotter, J. D., & Laursen, P. B. (2017). From lab to real world: Heat acclimation considerations for elite athletes. Sports Medicine, 47(8), 1467–1476. https://doi.org/10.1007/s40279-016-0668-9
6. Périard, J. D., Racinais, S., & Sawka, M. N. (2015). Adaptations and mechanisms of human heat acclimation: Applications for competitive athletes and sports. Scandinavian Journal of Medicine & Science in Sports, 25(Suppl. 1), 20–38. https://doi.org/10.1111/sms.12408
7. Garrett, A. T., Goosens, N. G., Rehrer, N. J., Patterson, M. J., & Cotter, J. D. (2012). Short-term heat acclimation improves the determinants of endurance performance and 5-km running performance in the heat. Applied Physiology, Nutrition, and Metabolism, 37(2), 284–293. https://doi.org/10.1139/h11-178
8. Buchheit, M., Racinais, S., Bilsborough, J. C., Bourdon, P. C., Voss, S. C., Hocking, J., Cordy, J., Mendez-Villanueva, A., & Coutts, A. J. (2011). Adding heat to the live-high train-low altitude model: A practical insight from professional football. British Journal of Sports Medicine, 47(Suppl. 1), i59–i69.
9. Racinais, S., Cocking, S., & Périard, J. D. (2017). Sports and environmental temperature: From warming-up to heating-up. Temperature, 4(3), 227–257. https://doi.org/10.1080/23328940.2017.1356427
10. Zurawlew, M. J., Walsh, N. P., Fortes, M. B., & Potter, C. (2016). Post-exercise hot water immersion induces heat acclimation and improves endurance exercise performance in the heat. Scandinavian Journal of Medicine & Science in Sports, 26(7), 745–754. https://doi.org/10.1111/sms.12438
11. Stanley, J., Halliday, A., D’Auria, S., Buchheit, M., & Leicht, A. S. (2015). Effect of sauna-based heat acclimation on plasma volume and heart rate variability. European Journal of Applied Physiology, 115(4), 785–794. https://doi.org/10.1007/s00421-014-3050-2
12. Neal, R. A., Corbett, J., Massey, H. C., & Tipton, M. J. (2016). Effect of short-term heat acclimation with permissive dehydration on thermoregulation and temperate exercise performance. Scandinavian Journal of Medicine & Science in Sports, 26(8), 875–884. https://doi.org/10.1111/sms.12526
13. Gibson, O. R., Mee, J. A., Taylor, L., Tuttle, J. A., Watt, P. W., & Maxwell, N. S. (2015). Isothermic and fixed-intensity heat acclimation methods induce similar heat adaptation following short and long-term timescales. Journal of Thermal Biology, 49–50, 55–65. https://doi.org/10.1016/j.jtherbio.2015.02.005
