Core body temperature: the metric and sensor behind two Olympic gold medals?

Why core temperature and heat training are hot topics in pro cycling right now.

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Achieving peak form at the right time was once a dark art. Advancements in the understanding of human physiology and modern training methods mean achieving peak form is now closing in on becoming an exact science. 

Despite those modern advancements in training, one thing that remains out of a team and rider’s control is the weather. Understandably, only a few riders truly enjoy wet and cold conditions and many riders struggle with the physical and mental demands of racing in such weather. 

However, many riders also struggle when the mercury rises at the other end of the temperature scale. While this year’s Tour de France was somewhat cooler than previous years, the athletes racing road and mountain bike events at the Toyko Olympics have had to contend with much hotter conditions. 

For some, extreme heat can be more detrimental and even dangerous than cold and wet conditions. Teams, riders, and coaches are now acutely aware of the benefits of heat acclimatisation and the associated performance benefits.  

A since-deleted tweet from new Olympic gold medalist, Anna Kiesenhofer.

While heat training and heat adaption are in no way new, a focus on body temperature has seemingly exploded in the past year. That explosion is unlikely to slow down anytime soon following Anna Kiesenhofer’s win in the women’s Olympic road race and her recent (but now deleted) tweets on her heat training preparation pre-Tokyo games. Kiesenhofer’s Twitter following grew by more than 300% overnight following her win. Meanwhile, a core temperature sensor was also clearly visible beneath the jersey of men’s road race gold medalist Richard Carapaz in the closing kilometres of that race.

We also spotted a huge increase in the number of riders using core body temperature sensors at this year’s Tour de France.

This explosion in the number of riders recording and tracking core temperature is perhaps partly due to a new, much easier measurement method. Previously core body temperature measurement required the athlete to use an invasive, often expensive, or one-time-only measurement tool (e.g. sensor pills).

Now, two Swiss companies, Core and GreenTeg, have partnered to create a continuous and non-invasive core temperature sensor making the measurement much more accessible.  Core has created its core sensor of the same name by incorporating a thermal energy transfer sensor from fellow Swiss company GreenTeg. 

The Core sensor usually sits between a riders heart rate strap and their skin and uses the thermal energy transfer sensor to calculate core temperature. If you watched any professional bike racing this year, you might have spotted a small rectangular shaped sensor attached to some rider’s heart rate straps.  

Bora-Hansgrohe, Canyon-SRAM, Deceuninck-QuickStep, Astana-Premier Tech, Ineos Grenadiers, Qhubeka-Nexthash, Ceratizit-WNT and Movistar are all officially using the Core sensor, and we’ve spotted countless other men’s and women’s WorldTour riders also using the sensor. 

Measure it, train it

So what use is core temperature to WorldTour riders, and can riders change or train their temperature? Christopher Jones of Core explained the benefits.

“Since sports scientists and trainers have known about the connection between core body temperature and performance, there has been an interest in capturing this information,” he said. “When you get hot, your performance drops. It is as simple as gaining a better understanding of each rider and helping them to train to improve and to be able to race at their best.”

Understanding how and when a rider’s core temperature changes with intensity and environment is invaluable in maintaining performance in hot conditions.

Teams and riders are now monitoring core body temperature to better understand heat stress and assist with heat acclimation, heat training, and event strategy. 

Riders and coaches now understand each rider has a personal heat threshold, above which their performance decreases. This is why we hear of some riders thriving in the heat while others struggle. Heat training is said to help reduce heat stress and, in turn, improve performance and recovery.

Athletes have long known the benefits of heat adaption and acclimation before competition in hot environments and will often attempt to replicate or train in hotter climates if expecting extreme heat at a target event. Tom Pidcock referred to training in a heat chamber with heaters and a kettle to replicate the conditions expected in Tokyo. Annemiek van Vleuten and Demi Vollering trained in a heated room as well ahead of their road race.

These riders are not alone – many athletes are now supplementing their regular training with specific heat training sessions. Much the same as an athlete would fitness test for regular training, Core prescribes a “heat ramp test” for athletes to identify their “heat training zone” using the Core sensor, an indoor trainer, and an elevated room temperature. The theory is that athletes can then condition the body through exposure to this heat training zone. 

This graph shows power (yellow), heart rate (red), and core temperature (green) during a heat ramp test.

Heat ramp test

The test and subsequent results revolve around two checkpoints and each individual’s core temperature, power, and heart rate at these points. The athlete or coach performing the test records the heart rate and power output at checkpoint one which is the moment the athlete’s core body temperature reaches 38 °C (100.4 ºF).

The rider then maintains the heart rate from checkpoint one until power output drops by 20% of power output one. This is checkpoint two. The athlete again records the core temperature for checkpoint two. The physical test is over at that point, and the rider records the highest temperature post-test. 

The temperature recorded at checkpoint two is the athlete’s heat training zone. The athlete can then add training at this temperature into their heat block training plan.

No two athletes are the same and as such each rider’s core temperature can vary vastly even in the same race, team, and clothing.

Each rider’s individual heat response can vary vastly. WorldTour athletes have the support of professional practitioners in creating their unique plan, but a typical heat training block can consist of two to four weeks of targeted training with six to seven heat sessions a week. 

WorldTour riders add indoor heat training to the end of their regular outdoor training rides. These sessions have riders ramp up to their heat training zone and maintain this for 45-60 minutes. After this initial block, riders can maintain the benefits of this heat adaption with just two to four heat training sessions per week.  

“Behind the scenes, or under-the skin, the heat training has a physiological effect where it produces more blood plasma which is crucial for sweat production. Continual heat exposure creates a blood hematocrit imbalance and to compensate for an increase in plasma, more red blood cells are created. This increased blood volume benefits cooling as well as power production.”

Christopher Jones of Core on the benefits of heat training.

Just the words “increased power production” are enough to excite any competitive cyclist, but the increased cooling efficiency and heat acclimation could prove more beneficial for performance in extreme heat. I included heat training in my Everesting preparations earlier this year to help eke out a few extra watts. Given the weather, during an Irish winter, I spent a lot of time on the indoor trainer and hence it was easy for me to include some heat training days.

While I can’t point to any scientific evidence that my heat training in preparation for my Everesting actually made a difference, the growing number of athletes and world-renowned coaches now including heat training for some of the world’s best athletes suggests this new core temperature metric is one they place a lot of value in.

Other uses

Cooling strategies

A cooling strategy for stage 15 of this year’s Tour.

It’s not just in training that teams are using the new core temperature metric. Teams are now preparing heat management strategies to aid race performance. 

Understanding a rider’s individual core temperature response to increasing temperatures and efforts enables the rider and their team to take a proactive approach to cooling. This proactive approach can help control temperature rises much better than a reactionary approach to feeling hot, at which point it’s usually too late to reduce core temperature. 

With the Core sensor, athletes can view their core body temperature on their head unit screens and sometimes view temperature increases long before they can feel any hotter. This early warning sign gives the athlete more opportunity to preempt a performance-impacting core temperature increase. Interestingly, the UCI has now officially permitted core body measurement in competition, meaning this metric is likely here to stay.

Performance analysis

Just like with cadence, heart rate, speed, or power, teams can now see time in “core temperature zones” distribution charts.

Some teams and coaches are now using core body temperature as another metric for analysis in rider training and testing to understand environmental temperature impacts on performance. 

In performance testing, an athlete in a hot environment may not perform to the same level as he/she would in cooler environments. Understanding the core temperature response can allow coaches to account for the environmental impact. 

Likewise, coaches are looking at the possibility of tracking longer intervals and training sessions to core temperature rather than just power or heart rate. 

Clothing

Clothing manufacturers are also turning to core temperature measurement in the design and testing process of new kit. In conjunction with Core and using multiple sensors, clothing manufacturers can measure energy transfer in watts to assess garments’ cooling or insulating properties. 

Extreme heat was never going to be an issue in Ireland in March. But with 76 repeats on such a steep climb, overheating could still be an issue.

“In recent times, the focus for performance sportswear has centred around aerodynamics, which is usually a trade-off against the thermal properties,” Young said, “Our testing can help the teams and the clothing brands find the right balance between aero and thermal properties and with the right fabrics, tip the scales toward sports clothing that delivers both positive aerodynamic and cooling capabilities.”

I also used data collected from the Core sensor in my Half Everesting ride earlier this year to help make the final selection on which skinsuit to use for my full Everesting record attempt.


Live core body temperature measurement was already quickly becoming the hottest (pun intended) training metric of 2021. That trend is sure to continue after two gold medal-winning athletes in the Olympic road races were using and tweeting about the Core sensor. Expect to see even more Core sensors beneath riders jerseys in upcoming races.

For more information on heat training and how Olympic athletes are preparing for the Japanese heat this summer visit Corebodytemp.com

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