Core 1 Sensor review: optimize your performance on the trail with this handy heat training tool

Core 1 Sensor: first impressions 

The Core Sensor is a wearable device used by some of today's top runners and cyclists that tracks your body's core temperature while you work out, helping you to improve your performance in hot weather and at altitude without having to swallow expensive pills, which was previously the only way to monitor your core temperature.

Developed in Switzerland, this handy gadget weighs less than half an ounce (12g) and can be clipped onto a heart rate monitor strap or sports bra to monitor your temperature. The company says its accuracy is backed by clinical, lab and field tests.

As we explain in more detail in our article on heat training, you can use the Core Sensor to train in a certain core body temperature range (around 101.3°F / 38.5°C) to help your body make more blood plasma, which allows you to cool down more efficiently. Heat training lowers the temperature at which you start to sweat and ensures you lose less sodium through your sweat, helping to avoid hyponatremia.

Representatives from the brand recommend undergoing 10-12 heat training sessions using the sensor over a two-week period approximately six weeks before a race, then maintaining your blood plasma levels with two or three sessions a week. This allows you to adapt for a hot race and essentially mimics altitude training (though the effects are said to be more long-lasting than actual altitude training).

To use this device, you'll also need to download the Core app to your mobile device, then you can pair the Sensor with your GPS watch or heart rate monitor. When you're ready to hit the trail, simply shake it to turn it on then wear it under your armpit for the best readings. While you run, your watch will tell you your core body temperature so you can sustain your desired temperature or cool down as needed.

It's a surprisingly easy-to-use piece of tech, and the information it provides is interesting and useful, but it's also expensive and really only for those training for a warm weather endeavor or heading to high altitude. It's not obvious when it's fully charged and if it's losing battery, and you'll need to be really serious about the data to make it worth your while, but if used properly, it could turn a race-day meltdown into a success.

• Shop the Core Sensor

Core 1 Sensor: in the field 

I first learned about the Core Sensor when I was writing an article about heat training, but I've been sitting on testing it for a while. I first received it in Scottish winter, which meant the best way to test it was indoors on a treadmill, which is almost my worst nightmare. I dutifully brought it with me on a work trip where I planned to take advantage of the hotel gym and start testing it, but foolishly, I didn't get it charged or set up properly.

Before I got the chance to test it at my gym at home, I got injured, and then as the months went by, it slipped off my list. Now, the weather has warmed up a little, and I have a high altitude trek planned in a couple of month's time in the Himalayas. Knowing what I now know about heat training, I realized that this could be a useful way to help me prepare for altitude, so I finally got it on during my recent mild-weather trail runs and hikes.

Since I've been testing this, Core has released an updated Core 2 Sensor which is lighter, smaller and smarter, according to the brand.

Here’s how it performed:

Design

This piece of tech couldn't be simpler. It's a small plastic square about the size of four small postage stamps that's designed to clip onto a heart rate monitor chest strap or even a sports bra.

It doesn't have any buttons at all. You charge it using the USB cable provided and there's actually no way by looking at it to tell if it's charged, you have to check the app. When you want to use it, you just shake it. If it's charged, a green light comes on, but again, you can't tell what the battery level is without consulting the app. So it's very simple, but maybe a little too simple?

Ease of use

It took me an age to finally trial this gadget, but once I did I realized it's almost embarrassingly easy to use. I charged it up and downloaded the Core app to my iPhone. The app prompted me to pair the device using a QR code and then I just had to pair it with my Coros Pace 3 watch so I could use it on the trail.

I ended up googling how to pair it and using a YouTube video, it really only took me a couple of minutes to add the Core Sensor to my Coros app. From there I customized the home screens for trail running and hiking so that I can keep an eye on my core body temperature once I get moving. Now when I'm on the trail, I can see my temperature along with my distance and pace.

I definitely prefer wearing it on my bra over using a chest strap, but the downside is that it's easy to accidentally throw it in the laundry. I haven't yet washed it by mistake, but even though it is waterproof, I'm not sure it would survive that.

Performance

When I spoke to Core's expert endurance coach Brian Maiorano about how to use this device, he advised that I wear an extra layer to help me stay warm when I run, and just to head out at a medium effort pace.

For a lot of people I know, this would be difficult, but I actually love being a little warm and sweaty when I run – I recently showed up to a group trail running dressed in three layers and wore all three for the duration while my running mates were in shorts – so I quickly realized I might have an advantage.

I decided to take this out for the first time on a mild (nearly warm) spring day. Rather than bother with a chest strap, I attached it to my sports bra which was a bit fiddly (it will only work on thinner fabric), but once in place, it is very secure and I don't even notice it. Ordinarily, I'd have worn just a t-shirt at these temperatures, but I kept my Montane Fireball Lite jacket on and was able to run quite comfortably.

I kept an eye on my watch, and after about 20 minutes, my core temperature reached 101.8F (38.8C). It stayed there for only a few minutes when suddenly it dropped down to around 101.3 and I realized I had started sweating, and therefore had cooled down a little. I was able to maintain that temperature for the remainder of my run.

I have a sneaking suspicion that, because I'm always overdressed for running, I've actually been heat training accidentally for years (I also use the sauna several days a week), but since my first session, I've been using it regularly for trail running and even on my hikes, where I've found that with extra layers, I can maintain a good core temperature for more sustained periods.

The one area of performance that could use improvement is the battery. It holds a charge for up to six days as advertised, or for several sessions, but there's not an easy way of knowing if it's charged or not.

Data

The Core Sensor provides data on your core body temperature, skin temperature and heart rate, all of which you can view on an easy-to-read graph which shows how much time was spent in different temperature zones, not unlike heart rate zones.

I am also able to see my Heat Strain Index for each workout, a metric developed by Core that quantifies how hard my body has to work to stay cool during exercise. The HSI ranges from 0, meaning not at all, to 10, which represents a very high strain. During my first run, my HSI was 3.28, which meant that despite my extra clothing and mild weather, I wasn't having to work too hard to stay cool.

There is other data too, which I'm still getting to grips with, such as a chart showing my Thermal Load Trends, which measure the continuous time spent in a heat training zone, and an unexplained Heat Strain Score that I haven't figured out yet.

In the app, a lot of this data comes with links to learn more, and while none of it is overwhelming, it's definitely for the metrically-minded. That said, it's also distinctly more streamlined and focused than the amount of data you'll get from a GPS watch.

Value

Since I'm not actually training to run in the Javelina 100, I can't personally say that this has helped me adapt to the heat, though I'm looking forward to seeing how my trip to high altitude goes. I follow enough professional athletes online to know that heat training is widely used, and I can't see why this wouldn't be a really useful piece of tech for an elite athlete.

However, with a regular retail price of nearly $300, it also isn't cheap, and you can get decent GPS watches for less. Then again, for the serious competitive athlete, it may be a small price to pay to avoid heatstroke.

Core 1 Sensor: the bottom line 

The Core 1 Sensor is a simple-to-use piece of gadgetry that could provide valuable and actionable data to elite runners. It's comfortable to wear and could make a big difference to your training and performance under certain conditions, though it's a pricey piece of kit and yes, it means yet more tech on the trail. But if we're being honest, if it's good enough for Tom Evans, it's good enough for us.

Pair the Core Sensor with:

• How Advnture tests products