When you exercise in hot conditions, your body works harder to keep you cool. That extra strain can lead to an increased heart rate, sweat rate, and perceived levels of exertion, which will reduce your ability to perform at your best. 

It’s not all bad news though.

Deliberately training in hotter environments can help you build adaptations that boost your performance in both hot and cooler conditions. In fact, one study published in the Journal of Applied Physiology showed that endurance athletes who undertook heat training improved their time to exhaustion performance by a similar degree both in hot and relatively cool conditions. 

So, if we teach our bodies to become more heat tolerant, we’ll fatigue slower and race faster in all conditions. Which begs the question…

Should you be heat training?

The Falmouth Road Running Race in Massachusetts has witnessed an average of 15 cases of exertional heat stroke per year for the last 18 years. That means two in every thousand participants ended up in the medical tent with a life-threatening condition! 

You’re probably thinking it must be a brutally long race or take place in an inhospitable environment. But it’s actually relatively short (11.3km / 7 miles) and conditions are usually mild, albeit humid (average air temperature: 23.3ºC / 73.9ºF and 70% relative humidity). 

So, why is there such a high rate of athletes being taken down by heat illness? Researchers believe it’s the combination of high metabolic rates and high running speeds that runners can produce in the environmental conditions that likely puts them at an increased risk of exertional heat illness. 

The authors concluded that athletes undergoing better heat preparation for events - even when they don’t appear hot - would increase the tolerance and safety for this and many other events.

Ultimately, training in hot or humid weather ahead of a race is a natural “stress test”. When your body is under heat stress, it goes into overdrive and, like a response to any stressor, your body adapts to handle that stress better the next time. Think of adapting to the heat like you would building muscle from regular and quality resistance training. Once you’ve adapted, you’re less likely to overheat and fatigue quickly in hot conditions or even during intense efforts in any environment.

With a structured heat training approach, you can teach your body to better regulate its temperature, fight against the risk of heat illness, reduce fatigue, and handle higher workloads - ultimately leading to a performance edge.

What are the benefits of heat training?

The key benefits of heat adaptation include:

  1. Altered neurological plasticity: Basically, your brain can rewire itself to make you feel more comfortable in the heat, with the brain's thermoregulation centre becoming more responsive and staying more active. 
  2. Improved thermoregulation: is more sensitive to changes in sweating requirements to better control body temperature (e.g. as cycling power or run speed and gradient changes). Crucially, this results in less heat storage and improves your capacity for sweat loss. So, you can produce more power and run faster in hot conditions for the same body temperature, thus delaying the onset of fatigue.
  3. Increased blood plasma volume: More blood volume means you can sustain a lower heart rate for a given effort. This means you can handle the significant blood drawn to the skin surface for cooling when the mercury and/or humidity rises.
  4. Electrolyte conservation: According to research, your sweat may become less ‘salty’, although this change in concentration likely won’t drastically change your strategy (i.e. if you’re a salty sweater, you’ll still need strong electrolytes) and not everyone appears to see this change. The most significant change in sweat sodium concentration with heat acclimation appears to be in the least well-trained individuals. The more training we do and the fitter we become, we naturally receive some partial heat acclimation benefits - we’re effectively getting warm and using our sweat glands more frequently and harder than the Average Joe or Jane. At the Precision Performance Lab, we’re seeing that the folks who are better trained are likely to see no changes in sweat sodium concentration, probably due to their sweat glands already being more efficient from years of training and racing.
  5. Better mental tolerance: Getting familiar with being hot and sweaty can give you an advantage, physiologically speaking, but also psychologically. This means you won’t turn up to your hot event with your heart rate ~10-20 beats higher than you’re used to in cool conditions. Which, especially at the professional level, could be a huge advantage when compared to less prepared competitors. 
  6. Significantly reduced risk of exertional heat illnesses: Heat training teaches the cells inside your body to become more tolerant to higher body temperatures, mainly through the release of heat shock proteins, which reprograms your cells to continue functioning at higher temperatures. In addition to performing better, you can stay safer when racing in any conditions, particularly hot and/or humid ones.

When should you be heat training?

Heat training can be integrated into your training schedule at various points in your year, but some strategies might fit better than others depending on your goals and schedule.

Off-season or early pre-season:

If you’re starting a heat block early in the year, ensure you do some foundational aerobic/endurance training to ensure you can begin and handle a structured heat block. Going in cold after the off-season might not be the best idea. 

In an ideal scenario, this block should involve 2-5 weeks of focused heat training. This would help develop heat adaptation before a more targeted race preparation phase.

Think of this as your “heat foundation” for the year, not too dissimilar to how you do base miles for endurance.

Close to key races:

Many athletes will do a short, intense block of heat-acclimation training in the lead-up to an important event, especially if it’s going to be in a warmer climate than they’re used to. 

  • Plan for at least 5-10 days of consistent exposure to any heat block. Well-trained athletes can get away with doing heat work closer to race day
  • In general, pro athletes only need to factor in three days of recovery after a pre-race heat block
  • Whereas less well-trained athletes might benefit from a longer 5-6 days of recovery pre-event. This is to ensure you can recover (and importantly) adapt to the heat and feel fresh on race day. It’s worth discussing the optimum time for your heat training with your coach
  • If you’ve already done a block of heat work earlier in the season, you'll need fewer heat training days to get yourself back to “100% heat acclimatised” before a hot race as your re-acclimation time is reduced
  • If you’ve recently been well heat-adapted, you may only need 3-4 sessions. Aim to start these sessions around 10 days before the race
  • If you’re not recently heat-adapted, you’d ideally complete at least 10 sessions within 14 days. Aim to start these sessions ~21 days before race day. Make a note to reduce your training volume (and intensity) outside these first 2-3 heat sessions to help manage the added stress
  • Your regular training load metrics (TSS, CTL etc) won’t fully capture the added stress of heat training because they don’t account for the cellular stress of elevated body temperatures
How quickly will I lose my tolerance to the heat?:

The decay from heat adaptation occurs rapidly when exercise is ceased entirely. In most cases, it’s very unlikely you’ll stop exercising after regular heat training, but if you do, expect to see large reductions in heat tolerance (e.g. ~95% heat adaptation dropping to 42% heat adaptation after 12 days of no exercise and heat). 

The good news is that after a period of acclimation, re-acclimation is much, much faster. For example, what may have taken you a good 10 exposures might only take 4.

Image Credit: Dannen et al. (2018) ©

What if I don’t want to lose what I have?

After an initial adaptation block of ≥ 5 consecutive heat sessions, “maintenance doses” of 1-2 heat sessions per week can help you retain your new tolerance without overdoing it.

Think of it like strength training: once you’ve built a base, smaller top-up sessions can maintain your gains. Maintaining your adaptations to the heat over many weeks can be the key to reaping the benefits of heat training. This is especially true when you’re heat training to optimise performance in cool weather. It’s in these conditions that benefits similar to altitude training can be found. After prolonged heat training, we start to see increases in haemoglobin mass that are similar and sometimes higher (~4%) than what’s observed with 2-3 weeks of altitude training (1.1 - 4%). 

The ideal approach is to become a “heat machine” year-round by keeping some level of heat exposure in your routine so that re-acclimation is easier and faster. But, if that’s not realistic for you, remember that some is better than none.

How to structure your heat training  

While specific protocols can vary to align with your specific goals, there are some key boxes to tick for a session to be a heat session:

  1. Elevated internal body temperature: Use your environment and/or additional clothing to get yourself warm enough. Your training environment should be significantly warmer than what you’re used to. Aim for a consistent temperature above 30°C (86 °F), or use a heated room or hot climate if accessible. If not, you can simulate it by layering clothing (think full winter kit, but on the trainer at home).
  2. High sweat rate (for you): One of the key adaptations you want to see from heat training is a more responsive and higher capacity sweating system. Like a muscle, you need to train that system. As you get more adapted, you should start to notice that sweat doesn’t just appear on your chest, back and forehead, but you notice sweat pouring out of your arms and legs as well. This means your ability to cover your body with sweat (which is key marker of sweating efficiency) is improving. To put some numbers on it, ~50-65kg individuals should aim for sweat rates between 1-2L/h, and larger individuals weighing 65kg+ should aim for an excess of 1.5-2.5+L/h to help drive the body to adapt.
  3. Duration: You need enough exposure time to stress your thermoregulatory system - often 60-75 minutes of exercise in the heat is more than enough when combined with the rest of your cool weather training.
  4. Intensity: Use a percentage of your maximum heart rate as the target for the session. Unfortunately, this isn’t one fixed number for everyone in all conditions. As a guide, exercising at 75-80% of your maximum heart rate should get you in the right zone. When heat training in hotter or more humid conditions, you can reduce the target down to 70-75%. Similarly, when exercising in cooler conditions, you’ll have to bring the heart rate target up to receive the same thermal load. Part of the nuance of this process can be guided via a device like the CORE body temperature sensor, which can help take the guesswork out of intensity management with heat training.
  5. Ensure you don’t get too hot: Limiting any heat session to less than 75 minutes will help ensure you don’t overdo it. Sufficient adaptation occurs between 38.5-39ºC (101.3-102.2ºF), and so when you start getting too hot (> 39ºC / 102.2ºF), you create too much fatigue and your recovery and ability to adapt to the stress may be impacted.

Plenty of top-level athletes and teams incorporate heat training to maximise their physiological adaptations. From elite endurance athletes preparing for the Tokyo Olympics to professional cyclists aiming for Grand Tours with scorching stages, real-world practice echoes what the science says:

Don’t forget that these benefits can apply beyond individual endurance events. Team sports - where high-intensity sprints and repeated efforts are the norm - can see improved performance and recovery.

For high-intensity, indoor team sports with high energy expenditure from the combination of functional movements and running (e.g. HYROX), being able to cope with elevated body temperatures without slowing down could be a major competitive advantage.

A heat training case study

From our own data collection in the Precision Performance Lab, we’ve seen some huge gains in heat tolerance. Our very British Sports Scientist, Minty (who isn’t exposed to warm weather very often!), underwent heat training as part of his preparation for a very successful 2024 IRONMAN 70.3® World Championship in hot conditions in Taupō, New Zealand.

He completed 13 x 60-minute heat sessions across several weeks in average conditions of ~39ºC / 102.2ºF and ~45% relative humidity in the lab. 

For his first heat session, he managed to average a respectable 164 Watts for a heart rate of 156 beats per minute, coming to an external to internal load ratio (Watts/heart rate) of 1.34. 

In his final session, he managed to hold a whopping 252 watts at an even lower heart rate of 152 bpm. This meant his internal-to-external load ratio improved by ~42% up to 1.90 (Watts/heart rate). 

Visit the Precision Performance Lab

You can learn more about the theory behind this type of metric from Dr Stephen Seiler on the Fast Talk podcast and other talks he has done. 

The raw external to internal load ratio was normalised to a standard 23ºC/73ºF wet-bulb temperature (which accounts for temperature and humidity in one metric) to account for subtle changes in environmental conditions on different days. This was done by multiplying the raw score by the wet-bulb temperature of the session and dividing the result by the standardised condition (i.e. 23ºC / 73ºF wet-bulb temperature). 

Many racegoers reported feeling the negative effects of the heat with warm race conditions (~23ºC / 78.8ºF - ~64% RH), whilst Minty never felt hot at all during the race. He raced a personal best IRONMAN 70.3® time, despite the warm and sunny conditions.

Key takeaways

Heat training isn’t just about suffering in a sauna or layering on clothes until you’re drenched in sweat. It’s a strategic approach to build your body’s resilience so you can push faster, harder, and longer in both hot and moderate climates. 

A well-structured block of heat-focused workouts - preferably early in your season, then topped up with occasional maintenance sessions - can provide performance benefits on race day.

  • Strategic forms of heat training improve performance by improving thermoregulation, increasing blood plasma volume, and reducing fatigue. This provides benefits for athletes in hot and cooler environments
  • Ideal timing: an initial block of 2-5 weeks early in the season (after foundational aerobic training) and shorter targeted blocks 5-10 days prior to key events 
  • Each heat session should last ~60-75 minutes, at an intensity of around 70-80% of your maximum heart rate, aiming for internal body temperatures of 38.5-39ºC [101.3-102.2ºF] without exceeding this range
  • Regular heat exposure (≥5 consecutive sessions initially followed by 1-2 maintenance sessions per week) helps maintain adaptation over the medium term, reducing the extent of any re-acclimation block later in the season
  • Rapid loss of heat tolerance occurs after complete cessation (50% decrease in heat tolerance after 12 days), but re-acclimation is notably faster (requiring just 4 sessions, instead of 10 to get close to ~100%)
  • Heat training reduces the risk of exertional heat illnesses by enhancing cellular heat tolerance through the activation of heat shock proteins
  • Performance improvement included not just physiological adaptations but also improved psychological comfort and mental resilience during competition in varying climates

Further reading