Core Body Temperature Monitor

The Core Body Temperature Sensor claims to be the first wearable device that continuously and accurately measures core body temperature on the go, and you don't have to insert any probes. It's a non-invasive method that has enhanced my heat awareness. The app is also easy to use and provides additional learning resources, but the price means it's not for everyone.

While measuring core body temperature is not a novel concept, monitoring it continuously typically requires invasive methods such as thermometers, probes and pills, often in a controlled environment.

> Buy now: Core Sensor for £231 from Core

These days you can measure pretty much any metric while riding, such as heart rate, power, cadence, and glycogen levels. Now, thanks to this device, you can measure core body temperature too.

The Core body temperature sensor is aimed at anyone who trains with a purpose, and there are lots of relevant applications for it.

How does it work?

The Core is a heat flux sensor, which measures heat energy entering or leaving the body, then an AI algorithm converts this heat energy into your real-time core body temperature. The company says it'll even work under water, measuring temperature accurately while you're swimming.

While I have no way of testing the accuracy, I found that my core temperature went up and down at the moments I would have expected it to.

To wear the sensor, it can either be clipped to a heart rate monitor strap or sports bra – or, alternatively, stuck to your chest using one of Core's supplied stickers.

I opted for the former, clipping it onto my heart rate monitor strap and I found that it stayed in place well with the small clip provided and was unnoticeable.

For sports and athletic activity, it is recommended that you pair the Core sensor with a heart rate monitor as it claims to deliver greater temperature accuracy.

Why should I measure core temperature?

Just as altitude training stimulates your body to produce more blood cells, heat stress stimulates your body to produce more blood plasma, and with this sensor you can safely increase the intensity of your sessions in the heat based on how your core temperature is coping.

A common misconception is that a core temperature sensor would only be useful if you live in a hot country or are trying to acclimatise for a hot race, but increasing your blood plasma is beneficial to anyone who wants to get the most out of their training, in a sport that's dominated by marginal gains.

There are other applications too. Just as training with power involves riding at specific zones to prepare for a race, it's beneficial to train at the core temperature you'll experience on race day, typically around 38.5°C for most people. Monitoring your core temperature can assist in determining your heat zones and training safely within them.

Even if you live in a cold climate and spend most of your winter on the indoor trainer, measuring core temperature is still valuable. If you're doing interval sessions indoors, excessive heat can lead to reduced power output. Using core temperature data allows you to experiment with cooling and hydration strategies to optimise your training sessions.

What are the benefits of heat training?

The principle of heat training is that the body is conditioned to perform better when it gets hot, because when people get hot in sports, their power output drops because of the body diverting blood away from the muscles.

I spoke with Brian Maiorano, Coach Liason at Core, who talked about how heat training has several benefits, including increasing blood plasma volume, lowering the sweating threshold for quicker cooling, and reducing sodium loss in sweat.

Are the pros using them?

The Core temperature sensor was initially targeted at elite athletes and is used by many UCI WorldTour and Continental teams such as Soudal Quick-Step, Ineos and Lidl-Trek for both training and racing.

We reviewed the Supersapiens Glucose Monitoring System which is banned by the UCI for making racing too robotic, but Maiorano assured us that the Core sensor is unlikely to be banned by the UCI, as a big benefit of it is rider safety.

It enhances riders' awareness, signally when to initiate cooling strategies like ice in jerseys, ice slushies or, in extreme cases, when to ease off.

In use

The Core sensor can be used in various ways, whether you're a professional cyclist engaged in extensive heat training, preparing for a hot race, or, like me, monitoring heat awareness and pinpointing performance decline threshold.

As well as coming with clips and stickers for mounting the sensor, you'll also find a small USB charging cable which uses magnets to attach to the Core sensor to charge it up. Core claims a battery life of six days of monitoring, and I found this to be pretty accurate.

There is a green light that flashes on the sensor to show it's awake, but a slight negative for me is that this light doesn't indicate when the sensor is fully charged. You have to open the app, which then shows you the battery level.

For connectivity, the smartphone app paired with the Core sensor and my heart rate monitor seamlessly, and it was easy to set up and use. Usefully, a graph is plotted live on the app while wearing the sensor.

It was also easy to connect the Core sensor to my Wahoo Elemnt, and core temperature was displayed in the SMO2 muscle oxygen field so the unit label showed as SMO2% instead of °C.

To get the most out of the sensor you should start by completing the heat ramp test, like a functional threshold power (FTP) test to determine training zones. While an FTP test gives you power zones for structured training, the heat ramp test gives you core temperature zones for heat training. There are instructions on the app, online and on Core's YouTube channel on how to carry out the test.

In short, you warm up on the turbo and ride until your core temperature reaches 38°C. You then maintain whatever your heart rate is at this temperature for 45 minutes or until your power drops by 20% and your core temperature will continue to rise.

The test was easy to understand and not as hard as I thought it would be, as you don't have to ride at a high power to get your core temperature where it needs to be. Core did advise wearing warm clothing and no fan, so it was a sweaty affair!

My heat ramp test delivered a result of 39.3°C, giving me a Heat Training Zone of 38.8-39°C which defines a safe and effective core body temperature range for training that's individual to me.

Completing the heat ramp test also showed me how unsustainable it is riding when you're hot, as I only got around 20 minutes into the test and saw my power drop considerably while maintaining the same heart rate.

It was interesting to find out this zone, as prior to completing the heat ramp test I saw temperature values of around 39°C while taking part in a gravel race but didn't know whether this was sustainable for me or not. Knowing that my core temperature naturally goes up to around 39°C when racing reinforced the importance of training at this core temperature.

I also completed a session riding for 45 minutes in my Heat Training Zone, which was uncomfortable at first, but it surprised me how quickly you adapt. You can then re-do the heat ramp test to see how your body's ability to cope has improved.

Using the Core sensor made me more aware that your core temperature stays high even when you start feeling cold, like when descending. Your skin cools down, but your core temperature can remain high, which is important to remember before tackling the next climb, particularly during a race, which is why many professional cyclists use cooling methods such as ice or slushies.

Conclusion

Overall, the Core Body Temperature Sensor worked well and increased my heat awareness. I'm not sure how beneficial it would be to me in the long run, so I would say it proves most valuable for individuals with specific training objectives, providing additional data alongside heart rate and power to optimise training sessions.

With this information, you can safely increase the intensity of your sessions during heat training and better understand what temperatures you're likely to start suffering at during a race. You can then design and test hydration and cooling strategies tailored to you, as well as more accurately pacing races and warm-ups.

The app is easy to use and, usefully, you can view the data on some bike computers and smartwatches.

Verdict

Adds a useful extra dimension to your training, albeit at a cost