Heat Training and Adaptation for Endurance Sports
- SamSportsDietitian
- Mar 18
- 7 min read
Updated: Mar 20
Hot and or humid environments can ruin the performance of a well trained athlete. Through the course of this blog I'll explain why hot and humid conditions can prevent you performing; then I'll discuss methods to cope with hot and humid conditions.
Sections
What Is Happening In My Body When It Gets Too Hot?
Lee and Scott way back in 1916 noted in their research paper “It is a fact of common experience that a human being in a hot and humid atmosphere feels a disinclination to perform muscular work.” This isn't great if you are racing in a hot humid environment. But what is really going on?
Your body is optimised to exist at a very specific temperature. Between 37 and 38 degrees Celsius. This a delicate balance of warm enough for chemical reactions to occur faster than at room temperature, cool enough that proteins are not damaged by the heat and warm enough for many bacteria to be uncomfortable surviving at the temperature. For these reasons your body defends itself from big changes in body temperature.
Some of the ways your body defends itself from changes in temperature are to shiver (making heat through friction) or feel uncomfortable in hot or cold environments so you seek shelter from the temperature. In cold conditions you will notice your hairs stand on end, this traps a layer of air against your skin which can act as an insulating layer. In hot conditions more blood is directed to your skin and your sweat rate is increased. This reduces your body temperature by blood passing heat to the air. At the same time sweat evaporates from your body taking excess heat energy with it.
Both temperature extremes can be detrimental to performance, but I will focus on heat. There can be situations where your body’s ways of cooling itself are no longer enough to cope: your body’s core temperature rises, your sweat rate increases until you are dehydrated and you are forced to stop. What is happening in this situation?
Firstly, blood is directed to your skin to cool you down. This immediately has an effect on performance as there is less blood delivering oxygen and nutrients to your working muscles. If the temperature is above 37 degrees Celsius there can be no exchange of heat from your blood to the air. Additionally, you could be producing heat faster than your body can get rid of it into the air. This is because your body is not particularly efficient at converting nutrition to movement energy. Only about 25% of nutritional energy produces movement, the other 75% produces heat. You can see why you produce so much heat when exercising hard.
So, if air temperature is high, or there is hot sun shining on your skin, your body’s first method of cooling itself is rendered useless. I call this the first method, but the real first method is your brain makes you feel uncomfortable in the heat and seek shelter from it. However, hiding in an airconned room is not going to lead to the result you want, so this is equally rendered useless.
Your body’s next method is using sweat. 100% humidity means the air has so much water in it, no more water can evaporate into it. The closer you get to 100% humidity the less effective sweat will be at cooling you down. However, your body doesn't know this and will continue sweating until you are at a temperature it is happy with. This can lead to dehydration. As you become dehydrated you lose water from your blood. Like reducing a sauce when cooking, your blood becomes thicker and more viscous. Your heart finds this harder to pump and will increase its rate of pumping. You will notice as you become more dehydrated your heart rate increases. This also happens due to blood being directed to the skin.
To summarise, your body gets hot through environmental conditions and waste energy production. Your body tries to maintain a specific temperature by making heat feel uncomfortable, directing blood to the skin and sweating.
Heat is sensed by your brain. Your brain tries to make you cooler by making you feel uncomfortable. This will reduce performance by making it harder to use muscles.
Blood being directed to the skin can limit performance by reducing blood going to muscles and delivering oxygen and nutrients. Sweating can reduce performance by making blood more viscous and through the loss of electrolytes needed for nerve conduction to innervate muscles.
Sweat can reduce your performance by losing water from your blood. This means your blood is more viscous and your heart has to work harder to pump blood around your body. This reduces the amount of oxygen and nutrition delivered to muscles. It can also reduce the amount of oxygen delivered to your brain and lead to reduced cognition.
How Can You Train To Perform In Hot Conditions?
For heat training and adaptation for an endurance sport here are some suggestions:
Get used to heat. Your brain tells you to stop less.
Cool your blood with ice. Keep sun off your skin.
Replace fluids and electrolytes lost in sweat. Allow sweat to evaporate.
Each of the reasons heat slows you down, discussed above, can be addressed through training and preparation to limit the influence heat has on your performance.
The first factor is your brains perception of heat and it communicating to your body to slow down. By regularly training in heat your body becomes less stimulated and concerned by the heat and you will be able to exert closer to your potential. The adaptations take 7 -14 days. After which you only need to maintain the adaptations with exposure every 3 - 4 days. This can be done actively (training in hot environments) or passively (sitting in hot environments like hot tubs, baths or saunas). Training in hot environments is thought to be slightly more effective as it trains your brain to be able to perform in the heat at the same time.
The next factor to address is blood going to the skin to diffuse heat into the air. You can help this by reducing sun light hitting your skin. You will have felt on sunny days the shade is cooler than the sun, despite the air temperature being the same. This is because the radiation from the sun can heat our skin, but not the air. However, wearing clothes which block sun from touching our skin can stop us sweating effectively, which would also make us over heat. Some hot countries get around this by wearing baggy white clothes, but these are useless for racing. Therefore, look for a breathable top, preferably white, which can reflect sunlight. Some brands have more shiny material than others. I suggest playing about with this to find a material which is breathable.
The final factor is the sweat rate. You can assess how much fluid you are losing by weighing yourself before and after sessions. You can also test how much salt is lost in your sweat with a sweat test kit. This will give you an idea for your nutrition plan and what you need to be replacing each hour.
You can try and reduce your sweat rate by keeping yourself cool. In Dan Bigham's hour record on the bike he reduced his core temperature by eating ice before. You will also regularly see endurance athletes in hot conditions covering themselves in ice. This will cool your blood if next to an area with high blood supply. Neck or groin are good areas to target for this.
Suggested Heat Training Protocol: for Heat Training and Adaptation for Endurance Sports
Do one session a day, five times a week for 2 - 3 weeks. For the first week only do your easier sessions. For second and third week add some higher effort sessions. You will likely have to adapt your target paces or powers to begin with.
To achieve target core body temperatures here is a suggested structure:
If under 10 degrees C: maximum layers, consider training inside
If 10 - 15 degrees C: wear leggings, thermal long sleeve
If 15 - 20 degrees C: wear a thin long sleeve
If > 20 degrees C: wear what you like
This is obviously just a suggestion and can be used for inspiration to develop you own protocol.
Warning: monitor your hydration status, heart rate and how you feel. If you feel unwell back off or stop the session. If you lose more than 5% of your body weight as sweat you can see clinically meaningful effects, so see this as a cut off.
When Should You Consider Heat Training?
I think you should consider heat training if your target event is likely to be hotter conditions than the training you have done. As I am writing this I am preparing for London marathon in April. Most of my training has been done in <10 degrees Celsius. Looking at average temperature for London in April (below) you can see temperatures of teens to twenties are not unlikely.
Therefore, I intend to do some heat training. For this I plan to run in a gym on a treadmill for 1 - 2 sessions a week wearing a light long sleeve and a cap. One of these sessions is a recovery run and I will build up to include a tempo run. The other heat sessions I plan are to have a hot bath after a training session. I believe I can tell if this is hot enough because I will feel the stress in my heart rate, hydration status and headache.
You can clearly see the effects of the heat in the photo below. My skin is more red, my eyes are sunken from dehydration and by the look on my face my brain is definitely telling me to stop.
The heat training sessions are also a great time to practice hydration strategies and train my body to absorb higher volumes of water while running.
If this article would help someone, please share.
Sources
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https://open.spotify.com/episode/7gyr1FiHfAUorvwzJSb6xy?si=9KqreZYzSGOgeeUYlCUKGQ
https://www.slowtwitch.com/training/training-adapting-and-racing-in-heat/
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