Reviewed: Core Body Temperature Monitor

Even if you aren’t competing in the extreme heat and humidity of places like Tokyo or Kona, being able to race and train in high temperatures are nearly an inevitability. In the past, figuring out how to best handle the heat has been somewhat of a guessing game: Dump ice, wear white, maybe acclimatize and adapt, maybe not. Things get harder when you take into account the ever-constant of human bodies—that we’re all completely different. Some people react well to the heat, some people don’t; some days you’ll have good workouts in high temperatures, some days you just won’t. While most smartwatches are starting to come equipped with dubious temperature sensors that read ambient temperature (and work ok) or temperature on your skin (not quite as useful), the reality is if you want to know your actual body temperature, you either need to run with a thermometer hanging out of your face, you need to ingest something, or the third option, which we won’t get into here. The good news is with the introduction of Core, you don’t need to do any of those things.

Core Body Temperature Monitor: The Basics

Up until now, there haven’t been any good solutions for those looking to accurately track and monitor their internal body temperature, and the fact that this device does so is a big deal. To be clear, there is a big difference between simply tracking skin temperature (which is not entirely useful) and your actual body temperature as you train, race, or just live. Core uses some technical wizardry to do a great job of getting both skin and internal temperature data into their app and the Garmin IQ app—the device also displays internal body temperature on compatible Garmin and Wahoo devices in real time via ANT+ and Bluetooth Smart. This allows you to monitor your body temperature as you train/race and after the fact (or throughout the day) on the app. From there, Core has protocols for heat training, active cooling, and more.

Core Body Temperature Monitor: The Good

The long-and-short of it is that the Core works quite well once you get it properly set up. If you are preparing for a particularly hot race and you have access to either hot training conditions or can create hot training conditions, this will help you see yourself acclimatize and let you keep yourself from flying over the edge. Of course, just like lots of data collection, there are caveats to how useful that info is, but we’ll get to that below.

We had great luck syncing the Core device to our Garmin devices as a data field, and the device itself is incredibly unobtrusive, assuming you’re used to wearing a heart-rate monitor. There is a sticky option, but we had less luck with that method of fastening.

As far as we could tell without getting…invasive…the temperature readings seemed accurate enough with never so much as a weird blip in data. This is a great start because any inaccuracies or dropouts would make this device not so helpful.

Core Body Temperature Monitor: The Meh

This is a tough section to write because by all accounts the monitor does everything it says it would and does it all very well. While we had some initial issues with a very very early version, and the firmware updates are a little bit of a tenuous waiting game, the information is always reliable and on all accounts accurate. However, in a slightly similar vein to the recent constant blood glucose monitors we’ve reviewed (like Supersapiens), the tough sell here is who needs this specific information, and what do you do with it once you have it?

The answer to the first question is actually pretty simple: You might need this information if you train or are planning on racing in a very hot environment. I would also add that you should want to race for performance or a time or a PR and not just finish. Yes, this could be used to prevent you from overheating if you’re just trying to finish a race, but you could argue that you shouldn’t really be getting close to that line if you’re not trying to win something, beat someone, or lower a personal best. But, if you already excel in hot conditions OR you don’t live/train/race in a hot place, there is not a ton of use for this device. At least not in how I’ve seen it used yet.

The answer to the second question is a little more complicated. Even if you’re prepping for a hot race where you want to perform at your peak or you train in high heat most of the time, there’s still quite a bit of unknown in interpreting the data that comes out of this device—no matter how accurate it is. The reality of internal body temperature is that for many people it rises and falls slowly, over many minutes—not sharply over seconds. It’s not the same metric as say your heart rate, which shows you relatively quickly when you’ve gone over the line (although of course it’s hard to know exactly why your heart rate has gone too high, as there are many confounding factors, but that’s another story). It’s also not the same as a power reading, which instantaneously shows you when you’re outputting too much or too little.

This means that your internal body temperature can change very very slowly in both directions—it can be hard to know when you’re on the path to destruction, and it can be hard to turn it around once it’s starting to happen (see my own graphs in a high temperatures/high effort situation below). The other dirty little secret is that the temperature at which you experience performance losses varies wildly from person to person, and the rate of change in internal body temperature does as well. In other words, to make the most of this device, you’ll need to do quite a bit of personal experimentation with little or no guidance to garner real takeaways. Put yourself out in nature with lots of other factors like wind, circadian rhythms, outdoor temperature fluctuations, and more, and the variables can start to add up—making meaningful interpretation incredibly tough.

Also, it bears mentioning that the app feels very much beta (it technically is), so I’m guessing that with more time, kinks in the software and analysis will eventually get ironed out and become more user friendly.

Conclusions

Much like the constant blood glucose measurement devices we mentioned before, tracking your internal body temperature in real time is an incredibly powerful tool for certain situations. But also like that device, it requires a lot of experimentation, personal knowledge, physiological knowledge, testing, and the removing of confounding variables. If Core was used in lab conditions or for very specific testing while training for a very hot race, it would be fantastic; out on the roads or trails, it’s a tougher sell.

Related: Understanding the Science Behind Glucose Monitoring

Since everyone is different, the readings will mean different things at different times in different conditions. I know my graphs above are certainly not typical findings—I perform well in heat, and I’m pretty well conditioned for high effort—so my very slow increase and decrease in temperatures may not look like everyone else’s. But because of that, it’s hard to interpret what to do with that info and how to positively or negatively affect even my own numbers coming out. But that’s part of the point. Because the fluctuations in physiology, conditioning, and conditions can make the data look and act differently, it’s hard to self-analyze without the help of an expert. Yes, Core works great for pro cycling teams with exercise physiologists and lab conditions, and yes, it worked well for people like Kristian Blummenfelt, trying to get to his limit for an anticipated hot Olympic Games. But is it for everyone? Probably not.