During a heat training session, you perform exercise under heat stress. You should feel hot and sweat heavily, and your Heat Strain Index should be elevated. You can induce heat stress by exercising in warm and/or humid weather, wearing warm clothing, or some combination of both. When indoors, you’ll want to limit air flow – in other words, no fans!!
Heat Safety
Before doing a heat session, please read the article Heat Safety. Overexposure to high temperatures can be dangerous to your health and can lead to heat-related illnesses such as dehydration, heat exhaustion, and in extreme cases heat stroke with potentially fatal consequences. To reduce the risk of overheating and dehydration, drink plenty of water before, during and after heat training. If you experience heat-related symptoms such as dizziness, nausea, numbness, tingling, muscle cramps, headache, confusion, and/or fainting, stop exercising and consult a health-care professional.
Pick a Warm Climate
Heat training without extra clothes is most effective when air temperature is at least 30°C/86° F. High humidity may allow lower air temperatures to be effective without overdressing. In cooler environments, you can overdress. But if temperatures are below 15°C/ 59°F, you may struggle to create sufficient heat stress. Ideally, the environmental conditions during heat training are similar to or exceed those you expect during the race(s) you are preparing for.
Or, Overdress
In cooler conditions, you’ll need to overdress (ie. wear extra clothing). Think of this as creating a warm and humid microclimate around your body – your own personal heat chamber.
It will matter what types of clothing you wear. For example, you don't want your upper layer to get soaked in sweat, as this will cool the skin. Experiment with what works best for you, but the following is effective for most:
Torso:
- A long-sleeved nylon shirt against the skin
- Followed by a waterproof layer (ie. rain jacket) to prevent sweat evaporation
- Followed by a thick thermal layer on top of the rain jacket (the rain jacket prevents the thermal layer from getting sweat soaked)
Legs:
- Nylon/spandex tights
Accessories (possibly needed in cooler air temperatures):
- Warm hat
- Gloves
Note: If ambient temperature is too cool, you may struggle to get sufficient heat stress.
The Heat Session
Most people do heat sessions while cycling or running, although heat sessions using other forms of exercise are entirely feasible. Indoor training is extremely popular for heat sessions, as it is very easy to control the climate and intensity.
Important: heat sessions should not be done while strength training, as overheating increases the risk of rhabdomyolysis during intense exertion.
Let’s get hot!
- Start the session with a short warmup.
- Then, increase your power/pace to a moderate intensity (75–90% of FTP for cyclists, half marathon pace for runners). This will raise your core temperature and Heat Strain Index relatively quickly.
- Your goal is to reach Heat Zone 3 (Heat Strain Index of 3.0–6.9). This may take 20–40 minutes.
Stay hot
- Stay in Heat Zone 3 (Heat Strain Index 3.0–6.9) for a continuous 30–80 minutes (45–60 minutes is most typical).
- To stay within this target range, you can adjust your intensity and/or clothing. Remember there is a lag between the time you make adjustments and when Heat Strain Index stops rising.
- Note that heart rate will rise even when power/pace stays constant. To avoid an excessively high heart rate, most people will keep heart rate constant and gradually lower power/pace. It’s not atypical for runners to walk toward the end of a heat session.
- Thermoregulation is highly individual. For some athletes, a Heat Strain Index of 3.0 will feel very challenging. Others may need a value of 5.0 to get good adaptations. For more information, see the article Individuality of Heat Zones.
- If your first heat sessions seem too challenging, try spending only 20 minutes in Heat Zone 3. Subsequent sessions will likely feel easier and you can gradually build up session duration and intensity.
- Don’t overdo it! Keep your core temperature below 39°C (102.2°F) and Heat Strain Index below 7.0, and limit the duration in Heat Zone 3 to a maximum of 100 minutes.
- You’ll be sweating a lot, so remember to hydrate! Dehydration does not increase the quality of the heat session, and may even be dangerous. Signs of dehydration: very dry mouth, no/little and dark-colored urine, ammonia smell of sweat, feeling lightheaded, headache.
- Tip: To estimate your sweat loss, weigh yourself before and after exercise, and monitor how much you drink (for water, 1 mL = 1 gram). Your sweat loss in kg = (body mass before exercise - body mass after exercise) + fluid consumed in kg.
Passive Heat Training
The information in this article applies to exercise heat sessions. Athletes can also achieve heat adaptations through passive heat training such as spending time in a sauna or hot bath. CORE has built a Passive Heat Training Load Calculator so you can manually enter your session info and count the exposure toward your overall heat load and Heat Adaptation Score. The CORE sensor should never be worn in passive heat training environments for safety and data inaccuracy reasons.
Passive heating is most effective when combined with exercise. It is recommended to do 20–40 minutes of sauna bathing (air temp ≥ 80°C) or hot water immersion (water temp 40°C) after your training session. NOTE: make sure that you build tolerance to these strategies gradually over time. To limit the decrease in blood pressure during passive heating, we advise to move your limbs every now and then. When getting up after the session, do it slowly to avoid fainting! Before doing passive heating, please read the article Heat Safety.
How to get heat adapted
To gain heat adaptations, you will need to do multiple heat training sessions. Read more about heat training schedules in the article Boosting Your Heat Adaptation Score.
References
Daanen HAM, Racinais S, Périard JD. Heat acclimation decay and re-induction: A systematic review and meta-analysis. Sports Med. 2018;48(2):409-430. https://doi.org/10.1007/s40279-017-0808-x.
Esh, CJ, Carter, S, Galan-Lopez, N. et al. A Review of Elite Athlete Evidence-Based Knowledge and Preparation for Competing in the Heat. J Sci Sport Exerc. 2024. https://doi.org/10.1007/s42978-024-00283-y
Heathcote SL, Hassmén P, Zhou S, Stevens CJ. Passive Heating: Reviewing Practical Heat Acclimation Strategies for Endurance Athletes. Front Physiol. 2018;20;9:1851. https://doi.org/10.3389/fphys.2018.01851.
Pryor JL, Johnson EC, Roberts WO, Pryor RR. Application of evidence-based recommendations for heat acclimation: Individual and team sport perspectives. Temperature. 2018;6(1):37-49. https://doi.org/10.1080/23328940.2018.1516537.
Racinais S, Hosokawa Y, Akama T, et al. IOC consensus statement on recommendations and regulations for sport events in the heat. Br J Sports Med. 2023;57(1):8–25. https://doi.org/10.1136/bjsports-2022-105942.