Train hot, race cold: the science behind the WorldTour's latest performance-boosting metric

Core-body temperature is the latest metric that looks like it’s taken a foothold in the professional peloton. We investigate why and reveal its potential use for recreational riders.

When Dan Bigham broke the world hour record in August, ticking off 55.548km at Switzerland’s Grenchen Velodrome, the Brit made much of the heat-acclimation work he’d undertaken in the build-up. “When it came to the thermal side, I wore two Core sensors and ingested a core temperature pill, too,” he told Rouleur soon after.

Filippo Ganna’s subsequent lowering of the record to 56.792km involved similar heat-acclimation work to manage the track temperature. “Core’s one of our team partners and we’re doing a lot of work with them around building this comprehensive thermal model,” Bigham continued. “Chris [Blomfield-Brown, Core product manager] lives about an hour from Grenchen velodrome, so was present at pretty much every training session we did there. He’s a really helpful guy.”

As it transpires, he is an extremely helpful guy as we tracked him down to find out if a metric known for heat-related work is of any benefit during a long, cold(-ish) winter. 

Popular in the peloton

The Core Body Temperature Sensor’s a square-shaped wearable device that’s positioned on the side of the torso. Ideally, it’s clipped to a chest strap or bra strap, although adhesive patches can be used for sleeping or everyday activities. If you looked closely enough, you’d see its outline through several riders’ tops at a hot race like the Tour de France. It works by measuring the thermal energy transfer moving from or into the body. This is then processed by an algorithm to calculate real-time core-body temperature and relayed to either a smartphone or a Core-twinned device like a top-end Garmin bike computer

This is unlike an e-pill that you swallow and then directly measures core-body temperature, a tympanic thermometer that measures your core via the ear or a rectal measurement. Costs and impracticality mean all three have their flaws during cycling. That, say Core, is where they come in. Their offering is unobtrusive and accurate. How accurate we can’t yet verify. We know of a Slovenian study that applauded its reliability but questioned its validity.

On Core’s part, their work with athletes around the world has resulted in them concluding that “our accuracy is a Mean Absolute Deviation of 0.21 °C when compared to the e-pill. However, our approach is to keep improving our technology so it will remain an ongoing process of gathering more data into the pool to get even better measurements”. 

What’s unequivocal is that it’s omnipresent across the WorldTour. “Which teams do we work with?” Blomfield-Brown asks, reflecting our question. “Well, I can’t say the unofficial ones but officially it’s Bora-Hansgrohe, Quick-Step, Movistar Team, Trek-Segafredo, Ineos Grenadiers… On the women’s side, Canyon-SRAM and Ceratizit WNT. If you take into account the unofficial ones, there’s probably not a WorldTour team that we’re not involved with to some degree.”

Image by Wout Beel 

Again, we can’t verify the unofficial claims, of course, but what we are aware of is that individuals like Bigham and his Ineos team have spent many hours poring over the raw data and comparing it with e-pills. The fact they’re still using it suggests it has its scientific merits.

Not just for the heat

It’s an impressive list. And one that’s seemingly more than marketing. WorldTour teams are sounded out on a regular basis by the latest, greatest transformative high-tech tool to gain a competitive edge. But this one’s gained traction. Why? More precisely, how are the teams using this Core sensor?

“One of its uses is the well-known one, which is acclimatising to the heat,” Blomfield-Brown explains. “As you spend more time training in the heat, your body stimulates numerous adaptations that are conducive to better performance in that heat. One is that your body improves its ability to balance how much blood it sends to the skin for cooling but still sending enough to the working muscles.

“Another cool thing that happens when you heat train is your body produces more plasma. Basically, it says that you need to sweat more. And the whole sweat mechanism starts working much more efficiently. Before his hour record, Dan spent hours sitting on a turbo trainer in a heat suit. He’d spin his legs at just 50 watts and he’d already have sweat all over his hands. His body’s pre-empting what’s to come.” This is important as by utilising the Core sensor, you can calculate when a rider might be acclimatised.

This is all well and good, you might ask, but those of us in Northern Europe are entering wintertime. We’re looking to acclimatise to coping with the cold, not taming the heat. Is the Core simply a summertime fling?

“Not a bit of it,” says Blomfield-Brown. “When your body produces more plasma, ultimately it produces more haemoglobin, too. That’s why you can enjoy performance gains in the cold by training in the heat.”

In theory, this is a big win. Haemoglobin is a protein in your red blood cells that transports oxygen around your body. Vis-à-vis, higher haemoglobin levels could mean greater capacity to distribute oxygen to your working muscles and so cycle faster, stronger and longer.

Blomfield-Brown’s bias is supported by independent research. Take a 2020 study by a noted team of Scandinavian sport scientists, including Bent Ronnestad and Carsten Lundby. In their study, five weeks of heat training raised the haemoglobin mass of 23 riders, leading to an increase in lactate threshold power output and 15-minute mean power output. The downside focused on the number of heat sessions required to elicit this significant effect, which was five one-hour heat-chamber sessions a week. 

“In our experience, that’s not necessary as indoor training’s arguably a natural heat adapter because you don’t have the windchill,” says Blomfield-Brown. “However, riders at WorldTour and recreational riders can both benefit from banishing the fan a couple of times a week. To crank up the heat further, we’re also about to launch Core suits. They’re like the ones Dan used in his acclimation training for the indoor record and they’re really effective.”

As you can see here, they look like a Tychem suit you’d wear to protect yourself when handling toxic materials. 

“As long as you do it safely – and that’s what the Core is about – we’d encourage training hot and racing cold,” says Blomfield-Brown. “I had a friend competing in a Rouvy race. He trained a lot indoors without fans. He then races with fans plus his son placed wet towels on him. He’s been riding for years but saw his normalised power rise by 4%. He said it was the cheapest performance gain he’d ever enjoyed!”

Find your zones

So, evidence suggests regular heat training will benefit you even if you’re riding outside in the cold. But at what core temperature are you looking at hitting to benefit from the sensor’s feedback?

Image by Lotto Soudal 

“It’s like training by heart rate in that we all have different thresholds of what delivers peak performance. Thermal regulation systems are individual,” says Blomfield-Brown. “What we know is, again like heart rate, if you train too low you won’t stimulate the necessary adaptations; if you train too high, your session will finish too early. I’ve seen riders that can operate at 40.5° and have little performance degradation. Then again, I’ve seen riders who can’t even reach 39.5° and fall apart.”

Studies have shown that “normal” core body temperature can stretch from 36.1°C to 37.2°C, so like power meters and heart rate monitors, you’ll have to do some legwork to work out your individual tolerances, albeit to ascertain at what temperature your core begins to start limiting muscular power. Core recommend undertaking a heat ramp test. Not only does this highlight where you start leaching power, you then have the benchmark to see how cooling strategies might impact performance.  

As that heat ramp link highlights, the Core app “allows you to track your heat training. For example, it shows you how much thermal load you’ve accumulated each day, week and month. This tells you if you’re doing enough to maintain your blood plasma gains”. Repeat the heat ramp test every four to six weeks to monitor heat adaption. “A heat ramp test done after a heat training block should result in a higher heat training zone than previously – this shows that your body can withstand a higher core temp without losing power.”

Pacing’s another area where the Core could pay dividends, says Blomfield-Brown. “If you know you have a climb coming up in 15km, do some active things to drop your core, whether that’s ice towels or how much you drink. It’s like pre-cooling for a time-trial. If one rider starts the bottom of the climb at 38.5C and another starts at 38°C, the one who starts lower could have an advantage because their performance won’t degrade as much (depending on what their individual thermal bandwidth, of course). This happened at a high-profile race last year but the team wouldn’t let me use it as a public example.” 

At the recent Ironman World Championships in Kona, Hawaii, many triathletes altered their pacing strategy based on their core temperature, especially on the run where lack of airchill means heat stroke is a very real issue.

I spoke to Kristian Blumenfelt’s coach, Olev Aleksander Bu, a couple months back and he was near fanatical about training by core temperature, saying the sensor not only saved thousands of pounds on e-pills but also led to the creation of the wafer-thin – and arguably X-rated – tri-suit the Norwegian wore to gold at the Tokyo 2020 Olympics. Its use stretches to clothing choice in the cold, too, says, the Core man, helping a rider to ascertain empirically when to remove or add layers at a race like Flanders. 

The modern-day cyclist, whether riding the Tour de France Femmes or the Fred Whitton Challenge, is loaded with numbers. Wattage, bpm, HRV… the list goes on. So, will another metric help, hinder or replace your current dataset? For the professionals, who have a team around them to digest and interpret the figures, and then advise, it’s not a problem.

For the recreational rider, well, that’s arguably down to your interest in technology and seeking that next gain. For a training device, the sensor’s not as fiscally prohibitive (£229.95) as some other tools, but remains a significant investment.

Either way, we’ll certainly be interested in where this technology heads over the next five to 10 years, especially as global warming is clearly a very real thing.

Cover image by Wout Beel