This wearable for pro athletes has me rethinking my whole approach to fitness training

Any athlete knows that training in sunny weather is harder, but thermal load and heat acclimation aren't things that everyday runners or cyclists think much about. Most of us seek out cooler or shadier conditions, drink water to cool off, and avoid the sun where we can. Simple! But after using with the CORE Body Temperature Sensor, I'm rethinking my whole approach to fitness training.

Unlike your fitness smartwatch's temperature sensor — which only tracks during sleep because it can't properly differentiate between your body temp and the surrounding air flow while on the move — the CORE Body Temperature Sensor clips onto an HRM chest strap (like my Polar H10) so it can rest snugly against your rib cage.

With this sensor, you can get continuous data on your body's core temperature (hence the name), with the data broadcast over BLE or ANT+ to your fitness watch in real time. So on my Garmin Forerunner 965, I can see my temperature data field right next to my heart rate and pace.

No doubt your next question will be, "But why do I need to know this?" That's what family and friends asked me when I described this sensor. 

The answer is somewhat complicated, but it boils down to one key fact: Just like runners need to worry about their "training load" to get faster and fitter, you also need to consider your "thermal load." And that's not something that your everyday smartwatch can quantify. 

Why heat training matters

Up until now, I've focused on heart rate zones, a pretty common metric. I aim for casual low aerobic runs, long high-aerobic tempo runs, or fast-paced anaerobic training in balance to improve my VO2 Max. But sometimes, it's difficult to understand why I perform better on some days while struggling on others. 

Since I went running with CORE's temperature sensor, I learned something that should have been obvious in hindsight: that my body experiences the equivalent of running a low-grade fever once I start running hard. A moderate workout keeps you under 100.4ºF / 38ºC, while my recent anaerobic track workout had me hit the top of the "ideal" workout zone (according to CORE) at 100.9ºF / 38.2ºC. 

Any higher, and you get into "Heat Training," or the more dangerous "Red zone" in which your performance plummets. I rarely came close to either zone, because running in winter naturally reduces any heat strain. But as the weather warms up, it's something you have to be wary of. There's only a 1ºF difference between the ideal zone to improve your fitness and a serious health risk.

That's why many Olympic or pro teams of cyclists and triathletes use CORE's sensor to train safely. I'm firmly on the casual side of running, but it was exciting (and a little demoralizing) to dip my toe into the world of pro metrics.

I say demoralizing because I've certainly slacked on my thermal load, which is why my performance seriously dips when I reach higher zones. 

According to CORE, training in Zone 3 — about 100.5ºF/ 38ºC — actually "enhances immune function, allowing the body to respond more effectively to viruses and bacteria." I'd love to see some scientific data to back this up, but it's a comforting thought that rigorous running makes both your cardiovascular and immune systems healthier.

The problem is that to see actual performance gains, you want to do 45–60 minutes of activity around 101.3º F / 38.5º C. That's not an easy temperature to hit, especially if you actively avoid warm conditions. 

"When enough thermal load is accumulated in this zone, the body will increase blood plasma and subsequently hemoglobin," CORE's blog post explains. "This makes more oxygen available to the muscles at all temperatures, increasing performance. The extra blood volume also helps cool the body in warm conditions."

Just like a higher VO2 Max means you can convert more oxygen into a higher energy threshold, a higher thermal load will ensure your body's oxygen use is more efficient in tough conditions. This is especially important for marathoners or cyclists who need to maintain a consistent pace for hours because once you crack the higher heat "zones," your ratio of HR to performance efficiency will never recover.

But if you're only climbing into this zone during race conditions, the heat strain of trying to cool off will only make you perform much worse than you do during training.

That's why I'm trying to use the CORE Body Temperature Sensor to make sure I actually climb into the higher heat zones, so my body can better withstand the heat in race conditions. That means finding the time for more indoor cross-training in warm conditions, or braving the California sun with a lot of sunscreen slathered on. Then I use the sensor to make sure that I'm hitting the right zone, without overdoing it. 

Should you buy the CORE Body Temperature Sensor?

The CORE sensor has plenty of upsides but also some drawbacks that make it a non-starter for some athletes. 

On the plus side, this sensor works with nearly every fitness watch thanks to BLE and ANT+ support. Depending on the device ecosystem, CORE usually has made some way of making your temperature data visible in real time on your watch, such as a Wear OS app and a Connect IQ downloadable data field.

In my tests, I found that the data is quite consistent and accurate (at least compared to my CVS thermometer). However, it occasionally gives you unreasonably low data if you don't seat it properly on your skin, so just make sure to check the numbers in the app first before you begin your run or ride, then give it a few minutes to acclimate to your body's data. 

The CORE app shows your core and skin temperature charts transposed next to your heart rate during workouts. You'll also find charts showing your daily Heat Strain and Thermal Load, plus your long-term average temperature and how your strain and load trends over time. For serious athletes, it has pretty much all the info you need.

Unfortunately for frugal runners, the CORE Body Temp Sensor costs a whopping $284.95, around the price of a smartwatch or a couple pairs of high-end running shoes. Unless you're a cyclist who needs constant temperature monitoring for hours-long excursions or a runner with plenty of weekly mileage, it's likely out of your league.

While it comes with some "medical-grade adhesive strips" to stick it to your body for long stretches, these aren't made for fast-moving, sweaty workouts. You need to be comfortable wearing a HRM chest strap it can attach to; you can technically use an arm strap like the COROS Heart Rate Monitor, but CORE admits that "the accuracy of the core body temperature measurements is lower" unless worn on the chest.

I think it would be especially exciting if CORE (or another brand) sold a chest strap with both heart rate and core temperature data packaged together, ideally with a slightly more reasonable price tag. But even though this sensor is arguably a niche tool for serious athletes, the idea behind it — training your body to better withstand high thermal loads — is much more important than most athletes realize.