Minttu Hukka Blog: Heat Acclimation for Athletic Performance

2023 was the warmest recorded year in the history of the world (1). The rate of global warming during the past 30 years has been much faster than the long-term trend: some locations have experienced more than 0.55-degrees Celsius increase in the average temperature per decade (1). Although large populations live in almost all annual-mean temperatures (2), it is suggested that human population densities might be, on average, higher in the temperate band (3).

It seems to be a common trend that major athletic championships are held not only during the warmest months of the year, but also in warmer and warmer and more and more humid conditions: for example, the most recent Summer Olympics in Tokyo (average daily high 30⁰C), 2016 Summer Olympics in Rio de Janeiro (average daily high 26⁰C); the World Athletic Championships in Doha (2019; 37⁰C), and Eugene (2022; 30⁰C); the Ironman World Championships in Kailua-Kona (30⁰C) and the FIFA World Cup in Brazil (2014; over 32⁰C  inside the stadium); and in Qatar (2022) where, despite moving the games to November-December, it was still 30⁰C with 60% humidity (4,5,6). This imposes a huge health risk and may be particularly problematic for many athletes from mild-to-moderate environments competing in these games as they are not naturally adapted. The World Athletics president Sebastian Coe voiced his concerns and suggested potential changes in the typical scheduling of the championships being held during the summer months (7). It is worth noting though that environmental heat stress can be prevalent in relatively temperate conditions and those previously tolerated by an individual (8). Exertional heat stroke imposes long-term health risks and has a recurrence rate of 10-30% (8)

What is Heat Acclimation Training, and what does it do?

Heat Acclimation Training (HAT) is intentional exposure to heat during exercise, usually performed indoors without air conditioning/flow to cool off the body and/or with extra layers of clothing; with the goal of improving the body’s tolerance to heat, and offsetting the subsequent decline in athletic performance (9). It is particularly useful for athletes residing in cold-moderate climates preparing for events in warmer climates. Heat acclimation and acclimatization are often used interchangeably when talking about heat adaptation, although their meanings are slightly different: heat acclimatization means naturally induced (e.g. seasonal) heat adaptation, whereas heat acclimation is artificial heat adaptation and the following physiological adaptation in response to repeated core and/or skin temperature elevations (10). For the purpose of this blog post, these terms are used interchangeably to refer to the body’s adaptation to heat.

HAT improves thermal comfort and physiological strain, and enhances aerobic exercise performance in heat. HAT also reduces the risk of serious heat illness during heat exposure. Biological adaptations related to heat acclimatization include integrated thermoregulatory, cardiovascular, fluid-electrolyte, metabolic and molecular responses. The magnitude of these adaptations greatly depends on the intensity, duration, frequency, and total number of heat exposures (10-13).

Adaptation to heat occurs after repeated exercise-heat exposures are sufficiently stressful to invoke heavy sweating and an elevated whole-body temperature. This typically requires at least 1-2 weeks of about 90 minutes per day of heat exposure (9,11). Most heat acclimatization procedures found in the existing literature were developed for occupational/military settings and not for competitive athletes (12). Thus, these protocols may not be suitable for competitive athletes who are fitter and participate in tasks that require higher metabolic intensities (9). There should be a gradual increase in the exercise intensity and duration, or just the length of heat exposure, each day of the heat acclimatization process (2). CORE body temperature sensor is a gadget that can help athletes to be more successful in their heat acclimatization process as well as exercising in the right intensity and maintaining the desired power output during training or racing in hot climates. Heat Training Zone is the core body temperature range optimal for heat training. This can be determined with the Heat Ramp Test - more information can be found on the CORE website (6). Typically, the heat training session should consist of 45-60 minutes in the Heat Training Zone, with the total activity duration of 90 minutes, initially 6-7 times per week (6). It is normal to have a lower power output/speed at a given effort level when exercising in heat, and the effort should be regulated by the Heat Training Zone (6). It’s worth noting that more is not better, and going beyond your optimal Heat Training Zone will not provide greater benefits. To optimize performance, the exercise-heat stimulus should as closely as possible mimic the expected conditions and demands of the competition (9). Heat acclimation is specific to the climatic heat stress (dry vs. humid) and physical exercise intensities during the exposure (9,13).

Heat acclimation and the following adaptations

Resting in hot conditions (e.g. sauna) or exercising in a temperate environment allow for some (limited) adaptations. Sauna or hot tub can be useful for increasing the core body temperature or maintaining the core body temperature after the heat training session for slower cooling (9). It is suggested that maintaining an elevated core body temperature right after the heat training session can help prolong the physiological effects of heat while saving physical effort (14), and positive effects on exercise and hematological values have been observed even when normal (non-HAT) training sessions are followed by sauna (15). Generally, it is recommended to avoid rapid cooling after HAT sessions as that can tamper the adaptation process (14). Studies suggest that heat training does not only improve performance in hot/humid conditions, but can also be beneficial to performance in temperate conditions (16,17,18). One main reason might be the increased plasma and thus, blood volume, which allows for enhanced delivery of oxygen as well as improved cooling mechanisms of the body.

Most adaptations are achieved during the first week of the HAT process (19,20). Heart rate (HR) development is the fastest and typically achieved in 4-5 days, and completed after the first week (19,20). Thermoregulatory benefits are generally completed after 10-14 days of heat exposure, and small additional benefits can occur afterwards (20). Adaptations will gradually disappear if not maintained with repeated exercise-heat exposure (9,19). The benefits are retained for about a week and about 75% will be lost by three weeks if the exposure is discontinued (11). However, reacclimating occurs quicker than the initial acclimation (21), and a few days spent in a cooler climate will not interfere with the acclimation process (9). After the acclimatization process is completed, every 2-3-day exposure should be enough to maintain the adaptations (12,13). Aerobically fit athletes are typically able to adapt quicker (by 50%), and the benefits will last longer (19,22). The main signs of heat acclimatization include lower HR and core body temperature; and higher sweat rate while exercising in heat (10,20,23). Lower skin temperatures, and earlier onset of sweating at lower body temperatures are also signs of heat acclimatization (9). Earlier and greater sweating improves evaporative cooling, and reduces body heat storage and skin temperature. Heat acclimatization also increases total body water and blood volume, as well as the conservation of sodium by sweat glands (11).

Table 1. Some outcomes and adaptations following heat acclimation. Modified from Sawka et al. (11).

As heat training increases sweat output, athletes should make sure they consume enough sodium, particularly early in the heat acclimatization process as salt deficit may lead to dehydration despite consuming plenty of fluids. Do not solely rely on the thirst-sensation: although improved after heat acclimatization, you will still likely under drink during the process (9). Keep in mind that there are individual differences in the total sweat volume and content, and the hydration plan should be tailored to meet those individual needs. Dehydration impairs athletic performance (read: Fundamentals of Hydration). An easy way to estimate sweat volume and content is by measuring your weight before and after training, and checking the clothing for visible salt coatings. If you can spot layers of salt on your workout clothes, you will likely benefit from extra electrolytes; a more precise method is to use the Nix Hydration Biosensor for determining the sweat content and getting personalized recommendations for when and how much to drink.

References:

1. Lindsley, R. and Dahlman, L. (2024, Jan 18). Climate Change: Global Temperature. Climate.gov. https://www.climate.gov/news-features/understanding-climate/climate-change-global-temperature
2. Klinger BA, Ryan SJ (2022) Population distribution within the human climate niche. PLOS Climate 1(11): e0000086.
3. Xu C, Kohler TA, Lenton TM, Svenning J-C, Scheffer M. Future of the human climate niche, Proc Nat Acad Sci. 2020;117: 11350–11355. pmid:32366654 
4. WeatherSpark. https://weatherspark.com/
5. N.p. (2014, Jun 29). World Cup 2014: Heat forces first cooling breaks in Brazil. BBC. https://www.bbc.com/sport/football/28075216
6. Sanderson, K. (2022, Nov 18). How will World Cup footballers cope in Qatar heat? Nature. https://www.nature.com/articles/d41586-022-03771-9
7. Elton, C. (2023, Feb 17). World’s biggest sports events may have to be held in cooler months due to global warming. Euronews. https://www.euronews.com/green/2023/02/17/worlds-biggest-sports-events-may-have-to-be-held-in-cooler-months-due-to-global-warming
8. Kenney, L. (2022) Exertional Heat Stroke: Updates and Controversies. [PowerPoint slides 6-9].  IRONMAN World Championships Medical Symposium. 
9. Hough, P. (n.d.). Heat Acclimation Training. St. Mary’s University Sport and Exercise Physiology Blogs. https://www.stmarys.ac.uk/blog/physiology/heat-acclimation-training.aspx
10. Taylor N. A. (2014). Human heat adaptation. Comprehensive Physiology, 4(1), 325–365. 
11. Sawka, M.N., Périard, J.D., and Racinais, S. (2016). Heat Acclimatization to Improve Athletic Performance in Warm-Hot Environments. Gatorade Sports Science Institute. https://www.gssiweb.org/sports-science-exchange/article/sse-153-heat-acclimatization-to-improve-athletic-performance-in-warm-hot-environments
12. Périard, J.D., S. Racinais, and M.N. Sawka (2015). Adaptation and mechanisms of human heat acclimation. Scand. J. Med. Sci. Sports. 25:S20-S38.
13. Sawka, M.N., S.N. Cheuvront, and M.A. Kolka (2003). Human adaptations to heat stress. In: H. Nose, G.W. Mack and K. Imaizumi (eds.) Exercise, Nutrition and Environmental Stress, Traverse City, MI: Cooper Publishing, 3:129-153.
14. Heat Training for Sporting Performance (n.d.). CORE. https://corebodytemp.com/pages/heat-training-for-sporting-performance
15. Stanley, J., Halliday, A., D’Auria, S. et al. (2015). Effect of sauna-based heat acclimation on plasma volume and heart rate variability. Eur J Appl Physiol 115, 785–794 (2015). https://doi.org/10.1007/s00421-014-3060-1
16. Kirby, N. V., Lucas, S. J. E., Armstrong, O. J., Weaver, S. R., & Lucas, R. A. I. (2021). Intermittent post-exercise sauna bathing improves markers of exercise capacity in hot and temperate conditions in trained middle-distance runners. Eur J Appl Physiol 121(2), 621–635. 
17. Scoon, G. S., Hopkins, W. G., Mayhew, S., & Cotter, J. D. (2007). Effect of post-exercise sauna bathing on the endurance performance of competitive male runners. Journal of science and medicine in sport, 10(4), 259–262. 
18. Minson, C. T., & Cotter, J. D. (2016). CrossTalk proposal: Heat acclimatization does improve performance in a cool condition. The Journal of physiology, 594(2), 241–243.
19. Pandolf, K.B. (1998). Time course of heat acclimation and decay. Int. J. Sports Med. 19:S157-S160.
20. Sawka, M.N., C.B. Wenger, and K.B. Pandolf (1996). Thermoregulatory responses to acute exercise-heat stress and heat acclimation. In: M.J. Fregly and C.M. Blatteis (eds) Handbook of Physiology, Section 4, Environmental Physiology. Oxford University Press, New York, Section 4, pp. 157-185.
21. Weller, A., Linnane, D., Jonkman, A., & Daanen, H. (2007)  Quantification of the Decay and Re-induction of Heat Acclimation in Dry-Heat following 12 and 26 days without Exposure to Heat Stress. European Journal of Applied Physiology, 102, 57-66.
22. Armstrong, L.E., and K.B. Pandolf (1988). Physical training, cardiorespiratory physical fitness and exercise-heat tolerance. In K.B. Pandolf, M.N. Sawka and R.R. Gonzalez (eds.) Human Performance Physiology and Environmental Medicine at Terrestrial Extremes. Benchmark Press, Indianapolis, IN, pp. 199-226.
23. Sawka, M.N., L.R. Leon, S.J. Montain, and L.A. Sonna (2011). Integrated physiological mechanisms of exercise performance, adaptation, and maladaptation to heat stress. Compr. Physiol. 1:1883-1928.

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Minttu Hukka is a professional triathlete and an exercise physiologist, with experience in wearable technology research. Minttu currently works at Huawei Technologies Finland as a human performance laboratory specialist, while pursuing her professional triathlon career.

Looking for guidance on planning heat acclimation for your key race or how to leverage sports technology for heat training? Our exercise physiologists are ready to assist. Book a free session with our expert for personalized advice on heat acclimation. Schedule your session here and now! :-)