Old Science, Modern Application of Heat Training

Heat training has become a popular topic in endurance sports, often framed as a newer performance strategy supported by modern research and technology. In reality, the idea of using heat exposure to improve performance has been around for a long time. Long before endurance athletes were using saunas, hot tubs, or environmental chambers, scientists studying physical labor and environmental stress were documenting how the body adapts to repeated exposure.

What modern research has done is not invent heat training, but clarify how and why it works, and how athletes can apply it more effectively. This paper examines the history of heat acclimation research, the physiological adaptations that occur with heat exposure, and how both passive and active heat training can be used to improve endurance performance in real-world settings.

Some of the earliest structured heat acclimation research came from occupational physiology rather than sport science. One of the most important early contributors was Aldo Dreosti, whose work in the 1950s focused on heat tolerance among South African gold miners. These miners performed physically demanding labor for long periods in extremely hot underground conditions, not unlike the stress experienced by endurance athletes racing in hot environments. Dreosti developed acclimation protocols that lasted between seven and fourteen days and grouped workers based on fitness and initial heat tolerance. His findings were clear and consistent. About fifteen percent of recruits were initially unable to tolerate the heat. After only four exposure sessions, nearly ninety percent of these workers adapted. After fourteen sessions, all recruits showed adequate heat tolerance. This research showed that heat intolerance is usually temporary and that the body adapts quickly with repeated exposure. These adaptations occurred without modern hydration strategies, electrolyte products, or cooling interventions. This early work demonstrated that heat tolerance could be trained and improved, providing an important foundation for later athletic research.

As research progressed into the early twentieth century, scientists began to better understand what was actually happening inside the body during heat adaptation. Studies from the 1920s showed that repeated heat exposure led to increases in blood plasma volume, lower heart rates during exercise, and improved temperature regulation. These adaptations reduced the overall strain on the cardiovascular system at a given workload. Further research in the 1940s by Sid Robinson and colleagues confirmed that these changes could happen in just a few days. One of the most important findings was the role of plasma volume expansion. An increase in plasma volume improves stroke volume, which allows the heart to pump more blood with each beat. This improves oxygen delivery and reduces cardiovascular strain, especially during prolonged endurance efforts. These basic mechanisms identified decades ago are still considered central to heat training adaptations.

Modern endurance research has taken these foundational ideas and tested how they apply to trained athletes. Passive heat training has become a practical option for athletes who do not regularly train in hot environments. Passive methods typically include sauna exposure or hot water immersion after normal training sessions. Research shows that one to three weeks of passive heat exposure can improve VO2 max, lactate threshold, and running economy. These improvements are not dramatic on their own, but they are meaningful when layered on top of consistent endurance training. One well-known study compared sauna suit exposure and hot water immersion to a control group and found that both heat groups improved VO2 max by about five percent. The hot water immersion group also showed better improvements in running economy. This suggests that full-body heat exposure may lead to more consistent adaptations.

From a practical standpoint, this is important because hot tubs and bathtubs are often easier to access and control than saunas, making passive heat training more realistic for many athletes.

Active heat training takes this a step further by having athletes train directly in hot conditions. This approach places stress on both the cardiovascular and thermoregulatory systems at the same time. Research supports its effectiveness. Cyclists training in environments around 95°F improved time to exhaustion by roughly nine percent after ten days. Runners exposed to repeated hot-weather sessions showed lower heart rates at submaximal intensities and improved efficiency. One of the most important findings across these studies is that heat adaptations carry over to cooler conditions. The increase in plasma volume improves oxygen delivery regardless of temperature, meaning athletes often perform better even when racing in moderate climates. This challenges the idea that heat training is only useful for hot races and supports its use as a general performance tool.

Longer-term heat exposure appears to help maintain and strengthen these adaptations. Research by Lorenzo and colleagues showed that athletes who trained consistently in hot conditions increased plasma volume and VO2 max even when tested in cool environments. More recent studies by Saunders and others suggest that heat adaptations fade over time if heat exposure is completely removed, but can be maintained with periodic sessions. This has practical implications for coaches and athletes. Heat training does not need to be limited to short blocks before hot races. Instead, occasional heat sessions throughout the year can help preserve adaptations. Additional research by Périard and colleagues has shown that repeated heat exposure increases the expression of heat shock proteins. These proteins help protect cells from stress and may support recovery and long-term durability under heavy training loads. This suggests that heat training may offer benefits beyond cardiovascular adaptation alone.

It is also important to separate heat training from other environmental strategies that do not provide the same benefits. Cold exposure, particularly frequent cold water immersion, has become popular for recovery. However, research shows that regular cold immersion can blunt training adaptations, especially those related to muscle development and strength. Cold exposure interferes with the signaling processes that drive adaptation. For endurance athletes training at very high volumes, cold exposure may provide short-term relief when recovery is the main concern. However, for athletes focused on improving performance rather than simply reducing soreness, frequent cold immersion may be counterproductive. This contrast highlights how heat exposure uniquely supports adaptation rather than masking fatigue.

Heat training remains one of the most consistently supported tools in endurance physiology. From early occupational research to modern sport science studies, the evidence shows that repeated heat exposure improves plasma volume, cardiovascular efficiency, thermoregulation, and endurance performance. While the tools and terminology have changed, the underlying mechanisms have not. Athletes do not need extreme protocols or suffering for heat training to be effective. Whether through passive heat exposure, active training in warm environments, or periodic maintenance sessions throughout the year, heat training can be adapted to fit the athlete, the environment, and the training phase. When applied thoughtfully, heat training is not a trend but a reliable, well-supported method for improving endurance performance across conditions.

Bibliography

Dreosti, A. O. (1955). Heat tolerance in industrial workers. South African Journal of Medical Sciences.

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.

Périard, J. D., Racinais, S., & Sawka, M. N. (2015). Adaptations and mechanisms of human heat acclimation. Scandinavian Journal of Medicine & Science in Sports, 25(S1), 20–38.

Robinson, S., Turrell, E. S., & Gerking, S. D. (1943). Physiological equivalent conditions of air temperature and humidity. American Journal of Physiology, 140(1), 168–176.

Saunders, P. U., Garvican-Lewis, L. A., Chapman, R. F., & Périard, J. D. (2019). Heat acclimation improves exercise performance in the heat and temperate conditions. Sports Medicine, 49(1), 1–17.