How effective is heat training?
It turns out that for me, a little bit of heat goes a long way.
If you’re at all tuned in to the endurance world, you’ll have heard all about the wonders of heat training.
Acclaimed coach Jason Koop wrote about this in his book, Training Essentials for Ultrarunning, back in 2021.
David Roche has been praising the benefits in his podcast for ages. And more recently in his prep for Western States, been cranking out heat suit miles on Youtube.
Even the GOAT, Killian Jornet has been heating up his home gym for training sessions.
And those are just the first two examples that come to mind. The list of professional athletes getting hot and bothered is endlessly abundant.
Wait, what is heat training?
I am not a scientist or doctor, so I’ll keep this top-level.
The premise is that by exposing yourself to heat - either actively or passively - you increase plasma volume in your blood, improve your sweat response, increase your heat shock proteins and boost mitochondrial efficiency. Amongst other things.
These adaptations improve thermoregulation, cardiovascular efficiency, and aerobic capacity, which equates to improvements in overall performance, improve fatigue resistance, and aid recovery.
My first experience with heat training was in the build up for the Addo Ultra in 2022, when my coach began prescribing midday runs to ‘heat acclimate’. These were not a whole lot of fun. But I didn’t suffer too badly on race day either, so they must’ve been effective.
I’ve dabbled in heat training since then - both active and passive. But if I’m honest, I hadn’t see a major improvement in anything measurable through my experiments. And perhaps that’s because I’ve never fully committed to a structured plan.
That all changed in the past three weeks.
Let’s take a step back quickly.
Heart rate variability (HRV) and Resting Heart Rate (RHR) are two metrics that are easy to monitor and are generally accepted as being associated with training readiness.
Neither are perfect, but with current wearable technology they’re what we have to work with.
Since the start of 2025 my HRV and to a lesser extend my RHR have been less than ideal.
My HRV has been erratic and mostly below average. My RHR has often been a good few BPM higher than normal. Both are clear signs that my body is unhappy, and that my nervous system is in crisis. My ability to train this year reflects as much.
By mid-March my body had enough, and my abused pelvis succumbed to a stress fracture. Initially misdiagnosed, I only found out in May, and continued to train (unsuccessfully) through the pain. And you guessed it, my HRV + RHR continued to reflect poorly.
Adding some heat.
Since 27th May, I full recovery from this injury.
No running, no riding, no core training. Only seated upper body exercises, which I have been doing every second day. I have also added a 15-minute sauna session after each workout. Nothing excessive.
Here is a screenshot of my HRV response in the past three weeks:
After two weeks, both my RHR and HRV improved dramatically. That jump from Orange to Green, directly correlates to seven days of structured heat exposure.
Throughout this period I have prioritised eating well, staying hydrated, and getting plenty of sleep. Sleep is something I am always an over-achiever at.
It can’t be overstated, that rest is playing a major role in the measured improvements above. But previous periods of full rest have not had the same effect. The only variable is heat.
I am not a doctor and don’t like to play one on the internet. I am purely reporting on my own experience. So I reached out some people with better credentials to add their input.
Here’s what John Middlewick from Carbon Endurance had to say:
“We know body goes through some good adaptions with heat exposure, the heat shock proteins and increased blood flow at lower intensities always works wonders for general recovery. I just want to see what some research shows on HRV.
I’ll come back to you. I’m always cautious, because people love taking what we say and then deciding they need to jump in sauna 4 times a week which could overload system if overall stress is high.”
Not wanting to punt fake-news, John reached out to Core Sensors for their input.
“I had a good chat with one of the sport science guys at Core temp sensors. They highlighted there isn’t really enough research at this stay to say whether or not active/passive plays a role in the effect of HRV changes.They acknowledge there are few studies showing positive changes in HRV which is positive and shows room for using HRV as a common marker in heat adaption research. There is some limited research showing that passive heat adaption protocols could help with positive changes in HRV. Suggesting that heat adaption protocols in general have positive effects on the body and its ability to manage stress.”
I also got in touch with Brendan Lombard, coach at Flat Rock Endurance and with a Sports Science Degree from Stellenbosch University. Here’s Brendan’s opinion:
I think more research is needed on the effects of heat training on HRV, however it makes sense that it would affect it positively.
In Matt’s case we must also look at this on a holistic level. Heat exposure most certainly plays a role, but if we pay attention to his psychological state as well as overall rest I am sure this too would send HRV values into the positive.
Dr Ross Tucker put it well - we need to triangulate recovery. Ask ourselves “How do I feel? What does my performance look like? And what does my physiology say?”
As wearables are becoming more of a “go to” for athletes, I do believe that one metric that can play a pivotal role in monitoring readiness is HRV. The accuracy and frequency of measurement just needs to become better as technology improves. History has, for a long time, attached wellness to heat exposure.
The Scandinavians have been doing it for ages. I think if an athlete can chase that 1% by having a sauna - why not?
What’s the conclusion?
Based on subjective experience, I feel that heat training has earned its popularisation in recent conversation. And in my case, the number don’t lie. Heat exposure has had a positive impact on my ability to recover.
It’s important to note that my dosage has been low to moderate. That’s because heat is a stressor, and I don’t want to create so much stress that it detracts from my ability to recover. At least not now.
As I continue to build back into training, this measured dose of heat exposure is going to remain part of my routine: 3-4 sessions a week, after exercise, for 15-20 minutes at a time.
Again, everything I’ve shared is subjective. Do your own research, ask experts you trust, and do some of your own experiments.
Thanks for reading. It’s been a busy week so this one was later than usual.
Tonight I am off to California for two weeks to attend TrailCon and watch Western States. It’s a dream trip, and I’m looking forward to sharing what I experience over there with each of you.
Until next time,
Matt
P.S. Thank you Nick Groll, for reading what I write and pointing out my typos so I can fix them. #RealFriends