Core is a useful wearable body temperature sensor for those battling heat training – but there are some fundamental snags to iron out.
Core Body Temperature Sensor is an unobtrusive way for athletes to how the your body is adapting to training in the heat. It's niche but it works, and the experience has improved to be more helpful. Ultimately though, our issues with the plastic build, app crashes and having to do a lot of the interpretation work means it's not quite the device we think is worth spending the big bucks on just yet.
Wearables that offer a deeper understanding of athletic performance are on the rise. We’ve seen sweat sensors breaking out – but Core promises insights into how heat affects your training.
Core is a wearable aimed at runners, cyclists, and anyone who needs help acclimatizing to hot or cold environments.
Unlike the many smartwatches and sports watches that now promise to track skin temperature, Core claims to monitor core body temperature reliably.
It’s a unique prospect that comes at a high cost, but is already being used by professional cyclists, ultrarunners and ironmen and women. But could Core have benefits for the average athlete? We’ve been testing it out to find out.
Design and build
Core is a very small, light sensor made from plastic. At just 12g is a device that once on, is easy to ignore and forget about.
Core provides some adhesive patches so you can apply it directly to your torso, or you can thread it through the strap of a heart rate monitor and use the provided black clips to keep it in place.
If you don’t use those clips when worn on a heart rate monitor they can fly off when things start to get sweaty. During a spin session I found the Core sensor on the floor next to the bike after the session when I didn’t use the clips. It feels like the sensor could be better designed to quickly clip to a chest strap without extra parts.
On the durability front, it’s a big positive to see it carries an IPX7 waterproof rating, which means it’s technically equipped to handle being submerged in water for up to one metre for 30 minutes. So it can handle heavy rain and sweaty downpours.
Though its ability to fend off moisture wasn’t a concern, the durability of the plastic sensor was, and it ended up snapping at the arms where it can be threaded into a heart rate monitor.
For something that costs over $200/£200, that simply doesn’t feel good enough to us. As a result we had to use some tape as a short term measure to keep it in place, but ultimately this is a problem for Core.
Along with the sensor, patches and clips, there’s a proprietary charging cable, so that does unfortunately mean if you’ve already got a watch or bike computer to track your workouts, you’re going to need to add another cable to your collection.
The port doesn’t clip to the sensor in an extremely secure way, so you need to make sure it’s well away from anything that could easily knock it out of place.
How does Core work?
It’s worth delving into the mechanics of the Core before getting into how it actually performed. The sensor technology used is a thermal energy transfer, that operates by detecting when heat moves from or into the body.
Core uses machine learning to take that process and convert it into the metrics and insights.
The companion app is available for iOS or Android, with a web app dashboard also available.
Some of those metrics can also be displayed on compatible sports watches, bike computers and smartwatches with the sensor offering both Bluetooth and ANT+ connectivity.
Devices supported include Garmin, Coros and Suunto watches, the Apple Watch and Wear OS smartwatches and it also works with bike computers from Wahoo and Hammerhead.
On a Garmin Forerunner 965 for example, we were able to view real-time core body temperature once we’d downloaded the Core Connect IQ app. We could additionally view Core’s Heat Strain Index, which aims to tell you about the amount of heat stress the body is experiencing.
Performance
From a connectivity point of view, the Core is pretty easy to set up. As mentioned, you have both ANT+ and Bluetooth connectivity and we had no issues pairing it to a Garmin watch, an Apple Watch Ultra 2, or using it with the companion app. It wasn’t always an instant connection with the latter and you’ll need to tap and shake the sensor occasionally and wait for the built-in LED to turn green to get things up and running.
While you’ll likely want to use it with a secondary device to view data, the sensor does also offer some storage capacity that does mean it makes room for around 2 days of data.
Core suggests you can use its sensor to monitor both core body temperature and skin temperature if you’re interested in seeing trends over the day. If you’re more interested in what it tells you about your exercise time, Core does also suggest using it alongside a heart rate monitor. This helps to improve the accuracy of the temperature tracking and the reliability of the insights you receive.
Core also says that you need to use the sensor for 10-12 sessions over a period of 2-6 weeks and those sessions need to be a minimum of 45 minutes to ensure the data and the insights are useful. This is another strong indicator that this is made for someone that’s going to be training for a good few months. You can do less training with it, but you ultimately want to be banking enough training with it on to make sure the data is useful.
I used the sensor in two stints, with the first in colder training conditions and the second one in warmer climes.
That time has been spent training for half marathons both in the UK and abroad and using it during training for a marathon, so the ideal type of training time. I also used it for indoor training time, throwing on some extra kit to get my body temperature up.
During my testing, the app has undergone a bit of a clean up and in the process has added some new insights.
In the app you have an overview of your data with a recording of your core body temperature alongside information on your heat strain, your heat training load and a new heat adaptation score.
This aims to simplify how you’re better coping with training in hotter conditions by scoring sessions on a scale of 0-100%. This is based on the heat strain captured in your heat training load, which is based on how much time you’ve spent with an elevated heat strain index. If that’s starting to sound complex, then it’s a bit of a window into our experiences with Core.
From an accuracy point of view, I didn’t have much issue with the core temperature tracking, which largely felt reliable. It didn’t feel wildly high or low when using in hotter and colder training time.
I compared real-time readings from the Core to a thermometer and readings never felt wildly out.
Using it for a half marathon in a cold Manchester wasn’t hugely insightful. Despite racing at intensity, my body wasn’t needing to work hard to cool it as the conditions were already pretty cold. It was a similar story when I tackled another half marathon, this time in Barcelona, and while conditions were slightly warmer, insights like heat strain weren’t of much use.
When conditions started to warm up or we’d thrown on more clothes for longer runs, the data started to kick into gear.
I saw higher heat strain scores and could see the temperatures where performance was declining. That correlation with performance is where using it with a sports watch or bike computer is really necessary to make the best use of the sensor.
Things like heat adaptation scores are things you can get a clear sense of your acclimation over time, but you do need a lot of the interpreting work ultimately. If you’re willing to do that, there are some useful insights here.
But an issue that plagued our experience was the number of app crashes, which seem to always happen when looking through the data analysis screens.
In terms of battery life, Core promises up to six days of battery life, which should cover you for a good week’s worth of training.
We found that on average, an hour’s use saw the battery drop by 4-5%, which seemed pretty good to us. Prior to the latest app update, it wasn’t clear if the sensor was still communicating with a paired device, which often meant the battery could just drain without you knowing.
Core has thankfully added a new standby mode that puts the sensor into a low power state when it’s not worn for 20 minutes. This mode according to Core can boost battery up to a month depending on how regularly you’re using it.
Should you buy the Core temperature sensor?
If you’re looking for something that can help you better understand your acclimation to training and racing in heat, Core is easy to set up and use and offers plenty of device compatibility.
It offers a range of insights along with good accuracy for core body temperature tracking.
Fundamentally though, the brittle design is a problem, especially given how much the sensor costs.
The recent changes to the app along with new metrics like the heat adaptation scores take away some of the heavy lifting that the user has to do to interpret the data it can capture.
The hope is that this type of monitoring is worked into sports watches and smartwatches in the same way that skin temperature tracking has. Until then, this is one of the few devices that can give you true body temperature data, but you need to be wary of the design and that you’re going to need to put the training time in with it to get the most valuable insights.
