CORE 2 Thermal Sensor… the same brain… but finally a body that doesn’t try to escape
I wrote a pretty deep dive last year on the original CORE sensor, the science behind heat acclimation, and why it can be such a big deal for endurance performance.
This blog is the follow-up… because I’ve now been using the CORE 2 and, for me, it fixes the one thing that stopped me recommending CORE 1 more widely…
It used to fall off. CORE 2 Thermal Sensor doesn’t feel like it will.
That’s the headline… but there’s more to it, so let’s get into it.
What is the CORE 2 Thermal Sensor?
CORE 2 is a thermal sensor that gives you real-time core temperature estimates, skin temperature, heat strain, heat zones, heat training load, and a heat adaptation score in the CORE app, and on compatible watches/head units.
The point is simple…
Heat training works… but most people guess it. CORE helps you stop guessing.
What’s new vs CORE 1?
CORE themselves are pretty clear… CORE 1 and CORE 2 currently have the same functionality and accuracy, but CORE 2 is smaller, lighter, and has a smarter physical design.
Key changes:
- 48% smaller and 30% lighter
- New modular clip (clip stays on the strap… sensor snaps in and out)
- USB-C magnetic charging cable (CORE 2)
- Battery quoted as 6 days, up to 30 days with standby
- IPX7 waterproof rating
The BIG win… the new clip design
On CORE 1… it came off my HR strap multiple times. Once I had to run extra miles trying to find it (I did find it… but still).
CORE 2… completely different feel.
The new clip design is a major improvement. It’s more secure, it’s easier to take on and off for charging, and I genuinely don’t see this falling off with the new clipping design.
If you ever “stacking it” in a race, you’ve got enough problems already… the last thing you need is losing a £250+ sensor in a ditch.
My use so far… treadmill heat suit… then hot tub straight after
Most of my sessions have been:
- Treadmill runs in a heat suit
- Straight into the hot tub for 20–45 minutes afterwards
Important note though… you shouldn’t wear the CORE sensor in a hot tub/sauna/steam room (data will be inaccurate and the heat can damage it). CORE recommend wearing it during the workout right up until you get in… then logging the passive heat session afterwards in the app.
So yes… I’m doing the hot tub work… but the sensor comes off before I get in.
Garmin Fenix 8 and the CORE Connect IQ app
I’m using a Garmin Fenix 8 with the CORE Connect IQ app/data field.
Pairing… no faff.
My main gripe is that it’s slow to connect to the CORE app and start giving readings. It gets there… it just takes longer than it should.
If yours is being properly annoying, CORE do have a troubleshooting guide (Bluetooth toggle, reset, etc).
Battery and charging
Real world… I’m charging it once a week.
That matches the quoted battery life nicely, especially if you’re using it regularly.
Heat training works… and I’ve got two real examples
I don’t need convincing on the concept of heat training.
I applied my own protocol properly in 2025 and it paid off twice:
- South Downs Way 100… around 25–26°C… finished in 17:54… felt comfortable the whole way
- Race to the Stones 100k… hottest day of the year (allegedly 35°C at one point)… came 3rd overall
Heat training improves heat tolerance… that’s obvious.
But it can also move the needle in broader performance terms too… there’s evidence of gains in things like plasma volume and haemoglobin mass in controlled settings.
Houston Marathon use-case (annoying…but useful)
My recent “race prep” use-case was Houston Marathon… I know, not an Ultra!!
Annoyingly my luggage got delayed and the sensor was in there… so I didn’t race with it. It was also cool weather until the last 20 minutes anyway.
Still… my Heat Adaptation score, according to the CORE app, going in was 66%, and I ran 2:57 despite a small niggle slowing me down.
CORE 2 didn’t run the marathon for me… but it’s part of the bigger picture if you’re trying to control variables and turn up properly prepared.
Do you need to find that extra 1%?
Here’s the honest bit…
CORE 2 is expensive. Sigma Sports have it at £264.95 as do CORE themselves on their own site
So, no… it’s not going to be high up my list for the everyday runner.
There are a load of other things you should be nailing before you even think about this stuff… training structure, consistency, strength work, sleep, fuelling, hydration… all the boring things that actually move the needle.
Heat training is also stress… and when you add that on top of training (which is stress) … it can knacker you out if you if you’re not sensible. This is especially pertinent in that heat adaptation is an acute intervention… it happens relatively quickly, and we lose it very quickly. This means you need to do it in the weeks before a race, which is traditionally our time to taper and reduce stress/fatigue! So, it’s a fine balance.
But…
If you’re competitive, racing in warm conditions, or you know you struggle in the heat… and you’re looking for that extra edge… CORE 2 is a proper tool
Nick Hancock is a UESCA Qualified Ultra Running Coach and Performance Nutrition Coach, UKA licensed and has coached many people to successful ultramarathon races such as UTMB, Arc of Attrition, Ultra-Trail Snowdonia, Dragons Back, The Spine, CCC, Canal Corridor, Race to The Stones, Endure24, Chiltern Wonderland, Wendover Woods and many more
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