Athletes and coaches are always on the lookout for ways to increase the Return on Investment (ROI) they get from the long hours they put into training. They want an edge over competitors and improvement in the face of diminishing returns from their existing training methodologies. As a result, there’s been a lot of talk recently about heat training.
Heat acclimatisation (where you go and train in a genuinely hot environment) and heat acclimation (training in a simulated hot environment like the Precision Performance Lab) have only relatively recently been well and truly proven to improve subsequent performance in the heat. However it’s only really recently that people have started to look at heat training as a way to improve performance in temperate conditions too.
That’s despite the fact that there’s research going back to 2010 that seems to demonstrate a clear link between heat stress training and improved exercise performance in cool conditions.
More recently, new research demonstrated that prolonged (10+ exposures within 14+ days) heat training could improve your performance in both hot and cool conditions, in a similar way to altitude training...
How does heat training impact you?
Heat training is very likely to boost performance in temperate conditions through multiple mechanisms. First, as you train in the heat, your body responds to the dissociation between heart rate and running speed or cycling power (which I’m sure we’ve all experienced) and adapts, by increasing blood volume through expanding the plasma (watery bit) of the blood to reduce the impact (increase) on heart rate during exercise in hot and/or humid conditions.
A larger pool of blood helps with heat dissipation from the core to the skin and provides a larger reservoir of fluid to lose in sweat before cardiovascular performance is compromised. This is something that’s also closely linked to your hydration status and your body's sodium levels.
Heat training also re-sets your internal thermostat to run colder and you start sweating sooner. This means you have more room to play with, in terms of core body temperature rise, before that becomes a performance limiter, as can be the case in high intensity events where heat production from the working muscles can be extremely significant.
More people seem to adapt to heat training more quickly and reliably than they do to altitude training as well. Maybe that’s because we all evolved from common ancestors who came from the hot African Savannah. This, along with the relative ease of implementation compared with altitude training makes heat stress training a very attractive option.
Does heat training improve performance?
Having raced a lot in some very hot conditions over the years (including Hawaii, Thailand and Saudi Arabia), I think I always tended to see a correlation between the amount of time spent in the hot environment pre-race and how well I performed. That relationship was pretty clear cut; simply preparing in the heat makes you more likely to do well in it, and it takes up to 14 days to get fully acclimatised to a hot environment (if you’ve been living somewhere cool for a while).
We've been working with a range of athletes in the heat chamber at the Precision Performance Lab, from triathletes to racing drivers racing in the Middle East and Asia, and runners competing in the Marathon Des Sables. We tend to see some fairly dramatic and positive adaptions in terms of lower heart rates, lower core body temperatures and lower RPEs (ratings of perceived exertion - i.e. subjective notions of how hard exercise was) in athletes undertaking just five back-to-back heat chamber sessions over the period of about a week.
We consistently see that even a handful of one-hour heat exposures (at about 39°C / 102°F and involving low-to-moderate levels of aerobic exercise) are enough to kickstart the body into becoming a lot more efficient at dissipating heat and conducting exercise in tough conditions.
To put some of the gains into perspective, one of our resident guinea pigs Sports Scientists Chris ‘Minty’ Harris underwent some heat training in the lead-up to the 2024 IRONMAN 70.3® World Championships in Taupō, New Zealand.
- During his first heat session at ~39ºC (102ºF) / ~40% relative humidity (RH), his average heart rate was 156 bpm for 164 Watts of power on the bike
- After thirteen x 60-minute exposures exercising at around 78% of maximal heart rate in 39 ºC (102ºF) / 40% RH across about two months, he’d improved his power output to 242 Watts at an even lower heart rate of 152
- That translates to an increased work capacity relative to his heart rate of 51% (1.05 to 1.59 Watts/bpm)
You can read more about this idea of output-to-input ratios on Joe Friel’s blog and a recent paper proposing its utility in self-monitoring acclimation status. This output-to-input metric can be used as an indicator of fitness but importantly here, it demonstrates individual performance and heat tolerance improvements independent of raw power increases
I think that based on the evidence I’ve seen and the fact that researchers and elite athletes are increasingly interested in the use of heat training as a way to boost performance in cooler conditions, it’s something I would definitely consider experimenting with. If I was taking my racing and training at all seriously, I certainly would be building in some heat training into my plan.
How to start heat training
Perhaps the best thing about heat training is that you don't really need a mountain or an altitude tent, as you do with altitude training. We see time and time again that your body doesn’t really care how it gets hot, so you can achieve at least some of the adaptations by just training in more layers of insulating clothing, rather than having to go into a special heat acclimation chamber. Of course, there's a trade-off with using additional clothing instead of a special chamber.
Firstly, many individuals report low thermal comfort during heat sessions with additional clothing compared with environmental chamber sessions. This is because skin temperatures rise relatively uncontrollably and your power outputs or running pace tends to suffer.
Secondly, using a chamber and an experienced physiologist who has the experience to adjust conditions to target higher heat or mechanical loads (power outputs or run paces) as you progress through your acclimation block, will help to ultimately improve performance on race day.
Regular saunas, hot yoga sessions and even just repeated exposure to hot baths have all been shown to exert at least some influence on heat-related adaptations in the body too, so it really is a type of ‘training’ accessible to just about anybody.
So, heat training might be something you want to try tinkering with even if your race calendar doesn’t include a trip somewhere sunny in the near future.
Heat training safety advice
Of course, one important thing to note is that seriously overheating yourself can be very dangerous indeed. It’s critical that you’re sensible about it and make sure you’re not putting yourself at risk by training ‘hot’. Here’s some advice to bear in mind to stay safe…
- Don’t heat train alone to begin with. Get advice from a heat expert or your coach
- Ensure that you're starting sessions properly hydrated by preloading with a strong electrolyte drink
- Consider using pre-cooling techniques before sessions
- Add layers
- Consider using a device like a Core Body Sensor to monitor your core body temperature (remember, you're aiming for around 38.5°C / 101.3°F)
- Have plenty of fluids available during sessions
- Fuel with enough carbohydrates per hour to meet your energy demands
- Measure your sweat rate to understand your losses and dial your hydration strategy for the conditions
- Rehydrate by replacing 150% of your fluid losses in the first two hours post-session
- Re-fuel after sessions (ideally with 1.2g of carb per kilogram of bodyweight each four for the first four hours after the session)
Got questions about how to approach heat training? Book a free one-to-one video consultation with one of our Sports Scientists from the Precision Performance Lab.
