Poor man’s altitude

Ferenc Soma Kovács, Hungarian national record holder middle-distance runner, NCAA bronze medallist and Harvard-record holder

I first encountered the phrase “heat training is poor man’s altitude” in the United States. Coach Jared Cornfield, head coach of the Northern Arizona University cross-country team, told me that Olympic silver medalist distance runner Galen Rupp regularly performed treadmill sessions inside an altitude tent set up in his garage as part of his preparation for the Tokyo Olympics. According to Rupp, the temperature inside the tent was typically around 95–105 degrees Fahrenheit (35–40°C), which prepared him exceptionally well for Tokyo’s humid subtropical climate.

In fact, many Hungarian athletes preparing for the Olympics were advised by team physicians to perform indoor cycling sessions inside a sauna or spend time in the sauna after training. In recent years, heat training has become a major focus not only because of global climate change, but also because research has shown that it produces highly beneficial physiological adaptations.

During exercise, the body dissipates excess heat primarily in two ways: through sweating and through heat exchange between the skin and surrounding air (known as convection). From basic chemistry, we know that evaporation is an endothermic process — meaning it requires heat energy, which it draws from the body. In other words, sweating cools the body.

Heat training specifically improves this heat dissipation capacity. The body begins sweating earlier — at lower temperatures — and in greater amounts. In addition, plasma volume increases and blood vessel regulation in the skin improves, allowing the body to transfer heat more efficiently through convection. As a result of these adaptations, athletes are able to perform at the same intensity with a lower heart rate under identical conditions. They also lose less sodium through sweating, which plays a crucial role in maintaining fluid balance.

What may be even more important, however, is that athletes who prepare in hot environments may also perform better under ideal weather conditions — not just in the heat. This is primarily explained by the increase in plasma volume. Furthermore, scientific studies have shown that long-term heat adaptation protocols may even increase total hemoglobin mass.

Bent R. Rønnestad and colleagues concluded in a study involving cyclists that five weeks of training in heat-retaining clothing increased hemoglobin mass by 2.6%, while training in a heat chamber produced a 2.4% increase. Interestingly, the control group experienced a 0.7% decrease in hemoglobin mass.

Similar trends were observed during exercise performed around the anaerobic threshold. At a blood lactate concentration of 4 mmol/L, athletes training in heat chambers or heat-retaining clothing produced 4.9 ± 3.2% greater power output after five weeks, measured in watts.

So if it is clear that runners can improve not only in hot-weather races but also in overall performance through heat training, the question naturally arises: how exactly should this method be applied?

According to experts, a minimum of 10–14 days is required for heat training to produce meaningful physiological adaptations. The method is most effective when the body is exposed to sustained high temperatures every day or every other day.

Of course, it is not necessarily advisable to compress an intensive adaptation phase into just two weeks. For example, if an athlete is already in the middle of a key training block that largely determines the success of the upcoming racing season, specialists will usually recommend only 2–3 heat adaptation sessions per week. This protocol typically produces the desired adaptations over the course of approximately six weeks.

As a general rule of thumb, it is recommended to significantly modify only one major training variable at a time. For instance, if a runner is planning an intensive eight-week half marathon block involving a 20–25% increase in weekly mileage, it may not be wise to introduce heat training simultaneously, as it represents another major stress factor for the body.

To fully adapt to high temperatures, approximately 10–14 heat adaptation sessions are usually required. According to sports scientists, every day spent without heat training may reduce total heat adaptation capacity by roughly 2.5%. This means that if an athlete completely stops regular heat exposure, the acquired adaptations may gradually disappear within 5–6 weeks. To counteract this decline, many athletes perform 1–3 heat sessions per week as maintenance work.

But what exactly qualifies as heat training?

If a runner wants to experience measurable physiological benefits, they generally need to spend time in an environment of at least 30°C (86°F). Exercise performed in hot conditions should last for a minimum of 30 minutes. Most competitive athletes complete sessions lasting 45–60 minutes while trying to keep their core body temperature below 39°C (102.2°F).

In addition to all of this, paying close attention to proper hydration and electrolyte replacement is critically important. Fortunately, wearable biosensors are now available that can be attached to the arm and measure sweat loss in real time, allowing athletes to monitor fluid, electrolyte, and sodium depletion during exercise. The device transmits the data directly to a mobile application, enabling the athlete to see precisely how much fluid, electrolytes, and sodium need to be replenished.

However, during heat adaptation training, monitoring hydration alone is not enough. It is equally important for the athlete to understand the actual physiological strain imposed by training in hot conditions. This is where Polar’s Cardio Load feature becomes particularly valuable. Based on heart rate data, the system analyzes the cardiovascular load of a training session — in other words, how much stress the workout places on the cardiovascular system.

In practice, this is especially useful because the same pace or power output can represent a significantly greater physiological burden in hot environments. A run that feels relatively easy may still impose substantial cardiovascular stress in high temperatures, something athletes often only recognize later through impaired recovery or declining performance. Within the Polar Flow system, however, it becomes clearly visible how Cardio Load increases after training in the heat, as well as how the body responds to this stress over the following days.

Based on my own experience, one of the greatest challenges of heat adaptation is how easy it is to cross the line where adaptation stops promoting improvement and instead becomes excessive stress. Polar’s recovery and training load metrics provide particularly valuable feedback in this regard, helping athletes objectively assess whether the body is genuinely adapting to the heat load — or drifting toward overtraining instead.

If anyone would like to try Polar products, the code POLARDTC15 provides a 15% discount on selected products at mypolar.hu.

References:

Rønnestad BR, Urianstad T, Hamarsland H, Hansen J, Nygaard H, Ellefsen S, Hammarström D, Lundby C. Heat Training Efficiently Increases and Maintains Hemoglobin Mass and Temperate Endurance Performance in Elite Cyclists. Med Sci Sports Exerc. 2022;54(9):1515–1526.