Wearables are a hot trend for both athletes and non-athletes. The CORE non-invasive body temperature monitor came out to great fanfare in 2020. How does it hold up to independent validity and reliability testing?
For a thermophysiologist like myself, I have all sorts of tools in the lab for measuring temperature. This includes skin temperature probes, heat flux probes to measure heat flow across the skin, muscle temperature, and of course multiple ways to measure deep body temperature. Common to all of these probes, however, are wires. Lots and lots of wires.
About 20 years ago, a big advance came about in body temperature measurement with the commercialization of thermometric pills that you can swallow, and which will wirelessly transmit gut temperature to a monitor. This allowed scientists to start tracking body temperature out in the field, whether it be with cyclists, firefighters, or soldiers.
These pills are great, but come with challenges of their own. They travel through the gut, so the temperature may change as it goes. Of course, they don’t last forever in your gut, are not reusable (obviously), and they cost a pretty penny at $60-80 a pill.
So a holy grail for thermophysiologists and sport scientists is a non-invasive core body temperature probe, and one candidate came out in 2020 in the shape of CORE. It has since been adopted by many pro teams and endurance athletes to help improve their training for racing in the heat.
In 2020, a research group from Slovenia published the first independent validation study on CORE, and is the focus for this week’s video.
Reference
Verdel N, T Podlogar, U Ciuha, H-C Holmberg, T Debevec, M Supej. Reliability and validity of the CORE sensor to assess core body temperature during cycling exercise. Sensors. 21(17):5932, 2021.
Click the video to watch!
Transcript
We’ve seen in this series how body temperature is critical to our health during both rest and exercise. However, the big challenge is being able to easily measure it in the field.
In today’s episode, we’ll take a look at the validity and reliability of the CORE body temperature monitor, a new non-invasive sensor that’s becoming extremely popular amongst endurance athletes.
A valid and reliable system to measure core temperature is one of the holy grails for athletes, sport scientists, and occupational health and safety. In the lab, we can use invasive esophageal, rectal, or even arterial probes to directly and accurately measure deep body temperature during experiments. However, that is obviously not easily or safely possible in real-life settings. There have been many sites and systems used as an analog or indirect measure of core temperature, including forehead, ear canal, oral, and armpit temperature, but these are very prone to errors. A better system are telemetric pills that measure deep gut temperature, but these are not reusable and are costly with repeated use. In 2020, the CORE body temperature system came onto the market to wide publicity. This system straps onto your chest, and uses a combination of heat flux, skin temperature, and heart rate to calculate deep body temperature. It is now being used by many top cycling teams and triathletes. But a big issue is that it was marketed without any independent testing of its validity or reliability.
What are these two terms and why are they important? Validity asks whether a system tells you the actual correct value that you’re trying to measure, while reliability tells you whether you will get the same value if you test the same situation several times. As an example, if we’re measuring the height of Mount Everest using a GPS, it is valid or accurate if it actually measures 8,848 m. But a GPS can be reliable or precise even if it is NOT valid. For example, it might read 8,000 m every time you take a measure at the summit. A system like CORE needs to be BOTH accurate and reliable if it is to actually be useful for monitoring in the field.
A Slovenian research group just published a validity and reliability study of the CORE. Reliability was tested by 12 trained cyclists performing the exact same protocol twice. They rode for 60 min at a steady hard intensity in 19°C in order to raise core temperature to high but not extreme levels. For a further test with much higher body temperatures, 13 trained cyclists rode at a hard steady effort at 30.7°C, and also did a warmup and cooldown of 10 and 15 min.
Let’s first look at CORE’s reliability, which was done by comparing the data over the two identical temperate trials. The two trials are shown with the pink and purple lines, and agreement between the trials were very high, with a mean bias of just 0.02°C. This is promising and good news.
However, things were not as rosy with validity, where the CORE data was compared with the gold standard of rectal temperature. In Study 1 with moderate temperatures on the left, CORE overestimated body temperature compared to rectal temperature. By itself, this might not be a showstopper if the overestimation was consistent. However, take a look at Study 2 on the right. With much higher temperatures of 38.5-39°C, where heat illness can become a serious issue, CORE data flipped 180° and UNDERestimated body temperature. This is possibly a worse case situation, where CORE seems to read higher than reality in non-critical situations, but reads too low when a person is actually hyperthermic. Overall, ~50% of the CORE data was more than 0.3°C different from rectal temperature, which the authors concluded as an unacceptable level for validity.
Wearables, whether activity, sleep, recovery, or now core temperature, are all the rage among recreational and competitive athletes, and are also moving into medical and occupational settings. I hope that this video highlights the need to look before you leap into this brave new world.
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