Heat Acclimation for Swimmers

Heat Acclimation for Swimmers

As the summer of 2016 approaches, two things are on my mind: the Olympic Games and how darn hot it’s going to be in Florida in just a few weeks. At the warmest part of the afternoon, our young club swimmers who are running as part of a conditioning and athletic readiness program join me on the trails that surround the National Training Center. In the summer, it’s the time of day just before the rainstorms bring down the temperature slightly for the evening.

Our club swimmers run between 2- to 5-k before their afternoon swim practice. I’ve thought to myself recently whether the extreme heat provides any advantage to our young swimmers? With the death of Fran Crippen in 2010, the advantages could even save lives. A quick search on the topic reveals that heat adaptation has been shown over time to reduce resting core temperature, increase water retention, decrease perceived exertion, decrease the perception of heat, improve thermoregulation, decrease the risk of heat-related illnesses, and decrease cardiovascular strain (Periard, 2015), which all sound like great things, but I wanted to know if they mattered for pool swimmers who need to perform in the water under well-controlled temperatures or in open water where nature controls the temperature. I also wanted to know if those who only swam and did not otherwise exercise in the heat would gain the same adaptations.

Essentially, we aren’t sure. While not directly performing swimming activity, this question appears to be first asked in a testable manner in 1982 by Avellini, et al. In the study, three groups of individuals were tested for heat tolerance (3 hours of cycling at 49°C or 120°F!) before and after training on a bike on land or submerged to the neck in either cold (20°C) or warm (32°C) water. Those who either trained on land or in warm water were better able to tolerate heat in the subsequent test. Extending the logic, this means that swimmers are at a disadvantage because their bodies are never quite permitted to achieve the same rise in core temperature as land-based athletes.

To decide if this phenomenon actually translated to swimmers (training and racing in a prone position with their heads submerged), Bradford et al conducted research that was published at the end of last year in the Scandinavian Journal of Medicine and Science in Sports (Bradford, 2015). The team specifically asked if swimming in warm water could induce similar adaptations as performing land-based exercise in the heat. Unfortunately they found that it did not provide any benefit outside of the relatively subjective measures of reduced perceived exertion and perception of heat. Practically speaking, the temperature at which they maintained the flume was a sweltering 33°C (91.5°F)—not something readily attainable for many flumes (or practice pools). The hypothesis is that the relatively cool fluid medium transfers more heat through conduction and thus the swimmer does not need to sweat as much. Additionally, even if the swimmer did sweat more, there is a physical effect of the water pushing back on the sweat glands called hidromeiosis (Henkin, 2010).

Turns out that in part because of hidromeiosis, most swimmers are actually bad sweaters and actually have similar sweat rates and sweat electrolyte profiles to nonathletes (2010). This shocked me. As a nutritionist, I am bombarded by the industry that insists upon full electrolyte profiles in sports drinks and supplements. However, it appears that swimmers actually only tend to lose high amounts of sodium and not other minerals in their sweat. Because they are also not sweating nearly as much, it appears that those who only perform swimming may need a simpler electrolyte drink than those who also perform land-based activities in the heat. Further, though it is conjecture, I submit that swimmers who do not also perform land-based activity of some kind (in the heat) are putting themselves at heightened risk for heat-related illness when they are exposed to temperature extremes, regardless of the medium.

Other benefits of heat adaptation that we are just beginning to understand include increased endurance, increased red blood cell count, increased plasma volume, increased growth hormone, increased heat shock proteins (protein protectors), and possibly even improved neurogenesis, learning, memory, and focus. Certainly swimmers might benefit from these advantages, even if they are not life saving. A study published in the journal Temperature earlier this year highlights even more advantages: four weeks of high-intensity interval training in the heat induced an increase in lipid oxidation and no increase in protein oxidation (Souza-Silva et al, 2016). This means that athletes can lose fat and maintain muscle mass with this type of exercise. So long as athletes are able to recover from the sessions, they get better and more resilient from their cells outward.

It is as of yet untested, but it may appear that repeated exposure to land-based heat acclimation may confer benefits to those athletes who compete in the pool, but also those athletes who will compete in open-water events in warmer waters (though the absolute maximum temperature has been reduced following recommendations (Racinais, 2015; Tipton, 2014)). Certainly the increase in water retention and the lowered resting core body temperature would serve the athlete well as pre-cooling is a popular method of enhancing endurance performance. When combined with heat adaptation, performance further improves (Schmit, 2015).

While our club swimmers run to attain some of their heat acclimation, this is not a requirement. Additionally, heat acclimation activities actually only need to be performed for a few days to garner the majority of the benefits. The standard recommendations are to train in the heat such that core body temperature rises (typically an hour). This could be cycling, running, rowing, or any other land-based aerobic activity. Within the first 4-7 days of this, the majority of the adaptations have taken place. Ideally, two weeks of adaptation would reap all of the benefits with those benefits lingering without further stimulus for 2-4 weeks (Racinais, 2015). That particular approach appears to me to be lengthy and cumbersome for swimmers who are already practicing for several hours a day. Another approach was studied in runners who spent 30 minutes in a sauna four times per week for three weeks after their training sessions, seeing the same increase in water retention expected from the former protocol (Scoon, 2007). Even without conducting any exercise in the sauna, the runners still saw improvements in their time to exhaustion. It follows that those swimmers who have access to a sauna after their training can utilize it to improve their heat tolerance.

As the summer approaches, consider incorporating heat adaptation and other hardening activities to ingrain psychological and physiological resilience in your athletes. Without question, be sure to monitor your swimmers closely, ensuring they hydrate well and do not push themselves past what their bodies allow. Even with small, regular doses of heat (or a summer training camp in Florida), the adaptations conferred will make your swimmers better athletes and may even save them.

Karl Riecken is the Coordinator of Performance Testing at the National Training Center in Clermont, FL. He designs and implements the strength and conditioning programs for the NTC Lightning and for visiting athletes. He is a sports performance nutritionist and serves as an adjunct faculty for the University of Central Florida’s Sport and Exercise Science program.


Avellini, B. A., Shapiro, Y., Fortney, S. M., Wenger, C. B., & Pandolf, K. B. (1982). Effects on heat tolerance of physical training in water and on land. Journal of Applied Physiology, 53(5), 1291–1298.


Bradford, C. D., Lucas, S. J. E., Gerrard, D. F., & Cotter, J. D. (2015). Swimming in warm water is ineffective in heat acclimation and is non-ergogenic for swimmers. Scandinavian Journal of Medicine and Science in Sports, 25 Suppl 1, 277–286. http://doi.org/10.1111/sms.12351


Henkin, S. D., Sehl, P. L., & Meyer, F. (2010). Sweat rate and electrolyte concentration in swimmers, runners, and nonathletes. International Journal of Sports Physiology and Performance, 5(3), 359–366.


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 and Science in Sports, 25 Suppl 1, 20–38. http://doi.org/10.1111/sms.12408


Racinais, S., Alonso, J. M., Coutts, A. J., Flouris, A. D., Girard, O., González-Alonso, J., et al. (2015). Consensus recommendations on training and competing in the heat. (Vol. 25, pp. 6–19). Presented at the Scandinavian journal of medicine & science in sports. http://doi.org/10.1111/sms.12467


Schmit, C., Le Meur, Y., Duffield, R., Robach, P., Oussedik, N., Coutts, A. J., & Hausswirth, C. (2015). Heat-acclimatization and pre-cooling: a further boost for endurance performance? Scandinavian Journal of Medicine and Science in Sports. http://doi.org/10.1111/sms.12629


Scoon, G. S. M., 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. http://doi.org/10.1016/j.jsams.2006.06.009


Souza-Silva, A. A., Moreira, E., Melo-Marins, D., Schöler, C. M., de Bittencourt, P. I. H., Jr, & Laitano, O. (2016). High intensity interval training in the heat enhances exercise-induced lipid peroxidation, but prevents protein oxidation in physically active men. Temperature, 3(1), 167–175. http://doi.org/10.1080/23328940.2015.1132101


Tipton, M., & Bradford, C. (2014). Moving in extreme environments: open water swimming in cold and warm water. Extreme Physiology & Medicine, 3(1), 12. http://doi.org/10.1186/2046-7648-3-1


Karl Riecken

Karl Riecken earned a master's degree in Health and Wellness and Applied Exercise Physiology from the University of Central Florida. During his undergraduate years he was a cross-country runner but fell in love with triathlon before he graduated. Karl has worked for the sport performance departments of both USA Swimming and USA Triathlon, conducting research and helping to coach world-class athletes at the Colorado Springs Olympic Training Center. He has been an assistant coach for youth triathlon teams and for USA Triathlon Elite Development Camps. Karl now provides physiological testing and training for athletes and patients at the National Training Center in Clermont, FL. Karl holds certifications from the National Strength and Conditioning Association, USA Triathlon, USA Track and Field, USA Cycling, USA Weightlifting, and the Road Runners Club of America.