At first it’s not so bad. I pedal at my usual speed on my indoor bike at home and I feel warm but nothing abnormal. Ten minutes later, however, I’m about as uncomfortable as I can be during a training session.
I notice it first in my heart rate measured by my chest strap: 180 bpm. One-hundred-and-eighty? I would normally be at about 140 or lower. Then it’s my hands—they are covered in sweat. Sweat drips down my face and nose and onto the towel covering my handlebars. I reach down for my bottle and sip the water. Just 12 minutes in and this is disgusting. I want to unzip my jersey, take it off, but I don’t. I sweat.
It is not unusual to sweat profusely while on the turbo trainer. Normally I would do this in cycling shorts and often topless with the windows open. But on this occasion I was sweating on purpose. I had closed the windows, I had put on my winter outdoor training kit (tights, long-sleeve shirt, thermal underlayer) and I was targeting a temperature—I wanted to bring my core temperature up from a baseline 98.8F to about 102F and then I wanted to keep it there, sit with the discomfort, the sweat, the smell, the pain.
And that’s exactly what I did. To monitor my temperature I’m using a CORE sensor. It’s a small wearable device that you can attach to a heart-rate chest strap or via sticky pads that come with the device. After those first ten minutes, my temperature was about 100.4F and slowly climbing. By about 20 minutes in, I was closer to 102.2F and this is where I held it for another 10 minutes before I could take no more. My clothes were saturated.
I soon slowed my pedaling, unzipped my jersey, and sat up and gulped what was left of my water. Ten minutes in heat zone one (the lowest level of heat training, roughly 98.6-100.4F), and 20 minutes in zones two (100.4-102.2F) and three (102.2F-103F). That’ll do for a first go. I sit down on my sofa and look at the CORE app, my heat stress score: moderate. Just moderate!?
What on earth am I doing? I’ve seen friends pass out from heat exhaustion after rides in 86F heat, and I usually do all I can to avoid heat while training and yet here I am deliberately making myself hot. In much the same way cold water swimming and the Wim Hof Method became so big in the health and fitness world (both also built on the idea of adaptation), so too could its opposite, heat training.
The Norwegian triathlon team, including Kristian Blummenfelt, an Olympic gold medallist and the first person to complete an Ironman-distance triathlon in under 7 hours, are doing regular heat training. The Belgian Olympic champion cyclist Remco Evenepoel is also known to use heat training, while the three-time Tour de France champion Tadej Pogacar, from Slovenia, has been spotted wearing a CORE sensor. And what the top athletes do always trickles down.
In 2018 the hottest London Marathon yet took place. It was 75F, which doesn’t sound so bad if you’re sitting in the park with a cold beer. But running 26 miles is another matter, especially when you consider the ideal temperature for running is about 59F.
It was a busy day for medics as runners overheated, and organizers even installed water spray areas where participants could cool down. The New York marathon, held in November, also often has temperatures of about 75F. As temperatures rise around the world, heat training could become the norm for athletes.
“When you exercise normally your body will get warm and the harder and longer you go the warmer your body will get, so it’s not abnormal to see core temperatures exceeding 102F in normal environmental temperatures,” says Olav Aleksander Bu, the sports scientist and elite coach famous for his successes with the Norwegian triathlon team and the so-called Norwegian Training Method, a science and data-led metabolic approach to training.
“The warmer the body gets the body will start to prioritize cooling to protect you. The body is like a plumbing system, and typically the more blood towards the skin means less blood into the muscles, less blood in the muscles means less oxygen in the muscles.”
This cooling is immediately visible in anyone since your skin reddens as it fills with blood. Blood comes to the skin to cool down through convection. You sweat and the evaporation of sweat helps to regulate your temperature.
“When you do heat training you can increase your blood volume, and the benefit of this is you’re better at sustaining cooling and also supplying more oxygen to your muscles,” says Aleksander Bu.
The idea is that, if you train hot your body adapts and you can race cool. Heat training, believe fans, could prove invaluable, especially over the winter while you prepare for the next summer’s events.
“It’s easy to get fascinated about the physiology and plasma,” Aleksander Bu says, “but it is as simple as when you track your performance you can see a higher pace and power output but for the same core temperature.”
So how is it done? There are two main ways for an amateur athlete: either bring the room temperature up to about 86F for indoor workouts, or layer up to produce a sort of personal heat chamber. Sorely lacking the money to whack the heating on, I opt for layering up.
Before you consider trying it, though, you need to ask: “Should I do it?” Heat training is not without its dangers.
“Heat training is for healthy individuals only,” says Brian Maiorano, the coach liaison with CORE. “Those who have a known illness or disease, or are under medical treatment, should consult a doctor for advice on whether or not heat training is right for them.”
He adds: “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.”
The day after my first session on the bike I go out running. The air temperature outside is 59F, perfect, but I put on my winter gear anyway. Running trousers, a thermal layer and a jumper. I do three miles and spend 20 minutes of that in heat zone 2 at about 101.3F.
The temperature is slow to rise but as soon as I’m in that zone 2 I have to fight every urge to take off my jumper but I leave it on. The sweat begins to saturate it and in no time I am back in an area of pain, my heart rate once again climbing.
But I find I adapt quickly, at least mentally. By my third heat training session, I already feel more used to the temperature. I am able to relax in the sweat and heat and my heart rate reflects that at least marginally. I take it slowly, I try not to ride or run at my usual pace but instead slow for the heat and I can train for longer.
“You’d be better off doing some heat training and bringing up the heat a little bit to see your temperature rise a little bit rather than going nuts,” Aleksander Bu says. “There are guidelines that suggest 101.3F is a good spot, but this differs from person to person. You’re looking to raise your body temperature 1.5/2C and maintain it there.”
On the last day of my week-long introduction to heat training I go out for a run in my normal gear. The temperature outside is 54F and I’m in shorts and a T-shirt. Clipped to my heart-rate strap is my CORE sensor. My temperature barely reaches 100.4F and it feels good not to be hotter. My average heart rate is about 15 bpm lower.
It seems unlikely that my body has already adapted to heat training, but just by pure contrast it feels better and I run better. I wonder if this is for every amateur athlete, and the answer is probably not—but I’m interested to see what will happen if I continue heat training, even if it is disgusting to do.
Sean Russell is a features sub-editor and writer at The Times of London
