CORE 2 Body Temperature Sensor Review: ‘Easy’ VO2max Boosts for Endurance Athletes?

The GreenTEG CORE 2 Body Temperature Sensor is an advanced, non-invasive device designed for real-time thermal monitoring, primarily aimed at optimising performance for serious athletes, including runners, triathletes, and cyclists.

Released in late March 2025, the CORE 2 builds upon its predecessor with notable usability improvements added to the same underlying technology. Its purpose is to provide athletes with precise, continuous data on their core body temperature, skin temperature, Heat Strain Index (HSI), Heat Zones, and Heat Training Load (HTL), which collectively help track their Heat Adaptation Score (HAS). By enabling safe and effective heat training, the CORE 2 supports any level to get heat-adapted and improve race performance in all conditions, with gains from haemoglobin mass, VO2max, lactate threshold, power output, and blood plasma.

This review will delve into the CORE 2’s design, functionality, performance, and value, with suggestions on using it and my experiences.

As an age group triathlete/duathlete with well over a decade of experience leveraging training methods, diet and wearable tech for performance optimisation, I’ve a keen interest in how to get faster. Easy claimed gains from zone 2 training in heat-stressed conditions interested me greatly.

Design and Build Quality

The CORE 2 represents a significant step forward in design from its predecessor. No longer are there fiddly attachments or worries about the charge status

It is compact and lighter, measuring 41.9 x 29.4 x 7.1 mm (without clip) and weighing 8.6 g (without clip), a 48% reduction in size and 30% reduction in weight compared to the CORE 1. This size reduction contributes to its comfortable and unobtrusive fit, typically attaching easily to a chest strap heart rate monitor.

A key improvement is the redesigned modular attachment system, including a pod that is detachable from its holder, which addresses previous user complaints about clip breakage and makes the device easier to secure and charge. The CORE 2 also features a new LED indicator that displays device status, battery life, and charging progress. In terms of durability, the CORE 2 is IPX7 water-resistant, meaning it can withstand immersion in water greater than 1.5 meters for over 6 hours, making it suitable for various training conditions. You can swim with it, but it won’t work when submerged in water!

As the following image shows, the Core sensor is worn on the side of the body. Take care not to cover the HRM’s sensing pads to the rear.

Setup and Ease of Use

While care is needed, setting up the CORE 2 is a straightforward process involving linking to the CORE app and your Garmin (or other compatible) device. The CORE app, available on iOS and Android, allows temperature and adaptation trends to be viewed, sensor settings to be managed, and firmware updates to be performed.

The initial setup for sports use takes 5 minutes of fiddling: You must pair your heart rate monitor to the CORE sensor via the CORE app for optimal accuracy and to enable Heat Training Load calculations.

Following this, the CORE Connect-IQ Data Field needs to be installed via the Garmin Connect-IQ app for Garmin users. Once installed, the CORE sensor is paired, and its ANT+ ID should be manually assigned in the data field settings if needed. Only one HRM ID can be used. If there are conflicts with other CIQ apps, make the Display Option setting shown above. Finally, the CORE data fields must be assigned to your activity screens on your Garmin device to collect and view real-time data. The process is generally intuitive for anyone who has previously used Garmin CIQ apps.

Other sports tech will pair directly to the Core sensor. Waho has now updated the pairing process to highlight CORE as a temperature sensor. There is still no ANT+ Profile for core temperature data. So, Core also broadcasts its data over the Muscle Oxygen ANT+ Profile. Thus, after you have paired Core as a temperature sensor with Wahoo, it will still appear as a muscle oxygen sensor, which is mildly annoying.

My main reason for not making enough use of Core 1 was that, oftentimes, when I came to use it, the battery was dead. Battery saving is one of many improvements made by the company, and now I have no excuse to avoid getting hot and sweaty! That said, getting it to wake up, pair with the app, and download data to the app can be a PITA sometimes.

The overall setup requires you to set aside a few minutes to get it right. Physically fastening the sensor to your chest strap is easy enough, but the app and CIQ need a bit of concentration, and everyone should be able to do it.

I used Core 2 on my Wahoo Roam 3 and Garmin Forerunner 970. Once set up, both worked flawlessly. Indeed, the Connect IQ app looks good and works well, and I was pleasantly surprised that Core got it working so quickly after the Forerunner was launched.

Features and Performance

The CORE 2 is scientifically validated by the company.

Stated Accuracy: The CORE 2 claims highly accurate core body temperature measurements of ±0.21°C. This accuracy has been validated against more invasive methods, such as ingested electronic pills, in laboratory testing. For intense efforts, a paired heart rate monitor enhances data accuracy.

Core & Skin Temperature Monitoring:

Measure Temperature> Determine Zone> Assess Load> Quantify Adaptation

The sensor measures skin temperature and models core body temperature near internal organs. It does this by adapting the skin temperature measurement by assessing your effort from heart rate and the temperature flux across the device. Validation of the algorithm to internally placed thermometers proves this works.

The metrics are displayed live on the CORE app and compatible sports tech devices. Core temperature naturally rises with exercise, with a time lag, and performance will suffer if it becomes too high. While varying with environmental conditions, skin temperature is vital for regulating core temperature, with cooler skin leading to a cooler core. The CORE 2’s ability to track these dynamically allows athletes to understand their body’s thermal response in real-time.

I’ll review Core’s metrics derived from the raw temperature measurements. Similar principles are applied for  TRIMP/CTL/ATL/TSB manipulations of heart rate based on zones.

Heat Strain/Heat Strain Index (HSI) & Zones – Your Real-Time Level of Heat Strain

The CORE 2 calculates a real-time Heat Strain Index (HSI), ranging from 0 to approximately 10, quantifying how intensely your body works to cool itself. HSI is an algorithmic combination of both Core and skin temperatures. A low HSI (0-0.9) indicates no thermoregulatory strain, while a high HSI (e.g., 10) represents extreme strain that could lead to health risks. Monitoring HSI live helps athletes prevent performance loss by making informed decisions on pacing or cooling strategies.

CORE categorises the HSI into four levels (zones), which you use during workouts to guide training and racing. As you can see, it’s not a case of keeping your core temperature below a simple level; the level changes depending on your skin temperature and personal factors.

Note: Heat Zone 3 (HSI 3.0–6.9) is identified as the ideal state for heat training, leading to quicker and more effective adaptation compared to Zone 2. While Heat Zone 3 is beneficial for training, it’s generally advised to avoid prolonged periods in this zone during competition due to increased heart rate and discomfort. Heat Zone 4 (HSI 7+) is not recommended for training due to potential health risks.

Note: Optimal training time in Zone 3 is 45-60 minutes

Heat Training Load (HTL) – A session or day’s contribution to your adaptation state

The Heat Training Load (HTL) is a daily value (0-10) that quantifies how much a training session contributes to heat adaptation. It’s calculated based on the duration spent primarily in Heat Zones 2 and 3. Aim for an HTL between 6 and 10.

Note: Core works as a ‘thermometer’without a heart rate monitor, but the HTL calculation requires HR data.

Note: 45 minutes in Heat Zone 3 typically yields an ideal HTL of over 6.

Note: The CORE app also features a passive Heat Training Load calculator, allowing estimated HTL from passive heating methods (like saunas) without directly using the sensor. Post-exercise passive heating can be particularly effective for boosting HTL. Even 40 minutes of passive heat training in a traditional sauna has a notable benefit.

Heat Adaptation Score (HAS) – Your Current Level of Adaptation

The Heat Adaptation Score (HAS) is a 0-100% metric that indicates your level of heat adaptation, based on accumulated Heat Training Loads. Consistent HTLs will increase your HAS, while days without heat training will let it naturally decline. Reaching a HAS of 25% marks “Heat Accustomed” status, 50% for “Heat Adapted,” and 90-100% for “Heat Champion”. Athletes can achieve “Heat Champion” status in at least two weeks with intensive daily high HTL sessions. Once adapted, maintenance can be achieved with 1-3 weekly heat sessions, aiming for HTLs around 6-12. The physiological benefits of increased HAS include lower core temperature, lower heart rate for a given workload, increased lower salinity sweat rate, greater comfort in heat, and enhanced performance in various conditions due to plasma volume expansion and potential haemoglobin increase.

• Thermal Rookie (0-24% HAS): Low adaptation, performance decline possible in heat.
• Heat Accustomed (25-49% HAS): Some physiological adaptations, but more training needed.
• Heat Adapted (50-89% HAS): Substantial adaptations, potential for high performance in hot races.
• Heat Champion (90-100% HAS): Very high adaptation, optimised endurance performance in all conditions.

My Heat Training Experience  & Real World Testing

2025 in the UK has been hot and sunny, probably the best ever year for training I thought.

I initially used Core for several weeks from April to the start of June, mostly cycling outdoors in warm to hot environments, hoping my regular training would boost my HAS. It didn’t. It had virtually no effect. I contrasted that to Garmin’s Heat Acclimation metric over a similar period, which showed 25% adaptation based solely on ambient temperatures (weather). I assume that Garmin’s number is wrong, at least for cycling.

I started again, taking it more seriously, running in hot weather and training indoors on Zwift/MyWhoosh without a fan. Core even sent me one of those Crime Scene suits – I used that once! This time, all aligned with what Core prescribes, but I found it difficult from a psychological and comfort perspective. I stuck with it for a few weeks, but dreaded most sessions, perhaps made worse as I head sweat quite a bit, and it was just unpleasant, even though the training itself wasn’t usually physically challenging, no threshold/VO2 stuff.

Example

Here is an example: I was pootling indoors on a KICKR for 40 minutes wearing a hoodie in Zone 1. It was about 24 Celsius indoors. Heat Strain was very slow to accumulate (there was a technical glitch at the start). I notched up the effort to Heat Zone 2 and HR Zone 3 (150bpm ish), then, the effort above that (Zone 4) quickly bumped the Heat Strain up into the target amber zone. I stopped after about 60 minutes, but kept recording, and you can see that the Heat Strain kept rising for quite a while afterwards to its peak at around 1:04:28.

At the end of this session, I wasn’t physically tired by a long way. I was, however, very wet. It was uncomfortable in terms of sweat, and a little bit uncomfortable with the heat from wearing a hoodie. The live stats are presented excellently on the Garmin with very clear Zoning and time-in-zone displayed; in the Wahoo environment, it’s simply a case of the live temperature. There is a well over a minute lag between exertion and a change in body temperature.

Example: Handily, my gym has a sauna, so I was able to tag a few half hours to the end of weights sessions and manually enter the various parameters into the Core app (personal biometrics + heating method (traditional sauna)  + duration + starting core temp. + air temp. + relative humidity). I found these sorts of ‘sessions’ considerably easier.

Combining these passive sessions with two heat sessions a week, my Heat Adaptation Score rose to 78% at one point. The 78% figure is slightly overstated as I incorrectly entered three passive sessions on the wrong days and couldn’t delete them in the app.

Training Takeout

If you can stand prolonged heat discomfort, Heat Training indoors wearing thick clothing seems to be a good way to raise VO2max

• Just because the weather is hot doesn’t mean you will have meaningful heat adaptation.
• Your regular training may well have very little heat adaptation. Core quantifies and perhaps contradicts your expectation.
• Cycling outdoors seems to have an (obvious?) cooling effect, inhibiting the effectiveness of heat training.
• Running outdoors seemed a more productive way to accumulate heat stress.
• The instantaneous heat strain value is a valid new data point for training. Its lagged effect needs to be considered.
• Passive training was a far more comfortable way to achieve what the app said were similar adaptation gains.
• Indoor heat training, when wearing clothing to raise the temperature artificially, was a sweaty and uncomfortable experience for me.
• The VO2max shown on my Garmin may be wrong, but it’s gone up 2.0 points in the last twelve weeks. I can’t say what contribution Heat training made to that rise, I was training a lot.
• The cumulative heat adaptation metrics offer helpful insights.

Here’s a video of me getting hot and sweaty using Core in a test – Google likes these things.

Core 2: Connectivity and Compatibility

The CORE 2 offers Bluetooth Low Energy (BLE) and ANT+ connectivity with excellent compatibility with the key sports ecosystems.

1. The Core sensor should be able to pair with just about any heart rate monitor to support advanced data. It won’t support newer secure connections.
2. The Core app works on Android and iOS.
3. The Core sensor supports, to some level, a wide range of sports watches and cycling computers from Garmin (via Connect IQ Data Field, compatible with 735XT and Edge 820 onwards), COROS, Suunto, Wahoo, and Hammerhead.
4. Post-workout data can be viewed and analysed with popular training software such as Garmin Connect. Synchronised temperature data is also supported by Today’s Plan, and Intervals.icu, Golden Cheetah, and the premium version of Training Peaks. You can perform more detailed analysis there.

Comparison with Alternatives

There is limited direct competition. To assess continuous core body temperature in a non-invasive way is not possible, AFAIK, except with CORE.

This table assesses CORE against alternative, inferior methods

Core 2 Value and Pricing

The CORE 2 Body Temperature Sensor is 269.95 CHF (Swiss Francs). This price point is consistent with its predecessor, approximately £220.95 direct or £294.95. A 30% trade-up discount is offered. It’s expensive for what it is, but I know of no direct competitor options. As a niche sports tool, there will only ever be a limited number of buyers, and the price reflects that. I’ve seen many similarly priced sports gadgets with little or no supporting science over the years. Core lays out the foundational science.

It’s a pro-grade product that supports training with significant, scientifically proven benefits. Pros will use the methods it supports without a thought. Its value to keen, amateur athletes all comes down to whether or not you can do the training.

Affiliate Disclosure: The item was free without payment or editorial control. At the time of writing, none of the links are affiliated. Please consider becoming a supporter to help the work here and produce more articles like this.

Core 2 review: Take Out

I trust the ecosystem to do what it claims. Can you trust yourself to do the training?

Skip? Not for casual athletes.

Buy? Serious athletes are committed to improvement.

In some respects, heat training is like altitude training, with similar positive effects. Core pointed out that some pro cycling teams incorporate both kinds of training.

Core 2 has a good level of usability, but there are quirks. The device itself works and pairs as expected; however, the quirks come from the app, which does its job but still needs improvement on how it links and syncs with the pod and on some of the data presentations and other messages. However, if you are analysing the data on different platforms, you will never encounter the Core app, nor will it affect the good in-exercise usage on your Garmin.

Other than the app, one area I would like to see extended is the use of Garmin Connect IQ. It would be great to see my Heat Adaptation Status as a widget on my Forerunner, perhaps indicating today’s contribution and the recent trend. I spoke with Core about this; it is possible, but would require a deeper integration with the logic in the Core app.

There are restrictions on the tech. The sensor won’t work in the water, nor should you use it for passive training in saunas. Whilst the ecosystem and its various calculations are deeply backed by scientific research, one aspect not yet incorporated is personalised Heat Strain Indices (ie we all react differently to heat). The company is working on that.

FAQs

Q: Is the CORE 2 accurate?

A: Yes, the CORE 2 claims medical-grade accuracy of ±0.21°C for core body temperature, validated against invasive methods. Pairing it with a heart rate monitor for best accuracy is essential for sports activities.

Q: Is CORE 2 compatible with my Garmin device?

A: Yes, the CORE 2 is compatible with Connect-IQ 2.4 or above, generally 735XT and Edge 820 onwards. It also supports devices from COROS, Suunto, Wahoo, and various training platforms.

Q: Does CORE 2 work for indoor training?

A: Yes. You can add fans or clothing to suit your level of comfort. The measured temperatures do not consider the ambient room temperature or outside weather; they will measure the cooling effect of a fan.

Q: What is ideal passive heat training?

A: 25-30 minutes at 85-90 degrees should be enough. Should feel uncomfortable or mildly discomforting.

Q: Will the Heat Strain Index adjust as my adaptation increases

A: Yes. It will automatically adjust with the progress of heat adaptation. Your core temperature will be lower for the same strain since your thermoregulatory system is better at releasing the excessive heat. Thus, the HSI will be lower. This is a distinct issue to the personalisation of the HSI between individuals.

Q: How long is the battery life?

A: It should last six training days longer if you enable standby mode.

Q: How long does it take to get a full charge?

A: 3 hours

Q: How does Core measure an internal temperature without an internal sensor?

A: The sensor infers it from your skin temperature, effort level and heat flux rate.

Q: Are Core 1 and Core 2 equally as accurate?

A: Yes, they use the same algorithms. Core 2 is a new version with improved usability rather than new algorithms. The flux rate through the new sensor differs from the old one, and the algorithms have changed to accommodate that factor. They were then internally validated back to the original data points.

Q: Is the Heat Adaptation Score like my VO2max?

A: No.

Q: How do I get VO2max increases with this training?

A: Keep your HAS above 80% for 4-6 weeks.

Q: How often should I do heat training to adapt?

A: To become thoroughly heat-adapted, the science suggests 10-14 heat sessions within a 2-12 week period, each including 45-80 minutes in Heat Zone 3 (HSI 3.0–6.9).

Q: How do I stay heat-adapted?

A: For maintenance, 1-3 sessions per week are recommended—ideally, 3×50 minutes.

Q: Without training, how quickly would my Heat Adapted status wear off?

A: 2-3 weeks

Q: What physiological benefits are there from heat training?

A: Properly executed heat training will lead to significant physiological adaptations, including increased haemoglobin mass, VO2max, lactate threshold, power output, and blood plasma volume. When heat-adapted, you should also experience an earlier onset and increased sweat rate, lower core temperature and heart rate for a given workload, greater comfort during heat stress, and a reduced risk of heat-related illnesses.

Sources & References

• Foundational Research (link)
• Flouris AD, Schlader ZJ. Human behavioural thermoregulation during exercise in the heat. Scand J Med Sci Sports. 2015;25:52–64. https://doi.org/10.1111/sms.12349.
• de Korte, JQ, Bongers CCWG, Hopman MTE, Eijsvogels TMH. Exercise performance and thermoregulatory responses of elite athletes exercising in the heat: Outcomes of the ThermoTokyo study. Sports Med. 2021;51(11):2423–36. https://doi.org/10.1007/s40279-021-01530-w.
• Nybo L, Rasmussen P, Sawka MN. Performance in the heat – physiological factors of importance for hyperthermia-induced fatigue. Compr Physiol. 2014;4(2):657–89. https://doi.org/10.1002/cphy.c130012.
• Périard JD, Eijsvogels TMH, Daanen HAM. Exercise under heat stress: thermoregulation, hydration, performance implications, and mitigation strategies. Physiol Rev. 2021;101(4):1873–979. https://doi.org/10.1152/physrev.00038.2020.
• Wingo JE, Ganio MS, Cureton KJ. Cardiovascular drift during heat stress: implications for exercise prescription. Exerc Sport Sci Rev. 40: 88–94, 2012. https://doi.org/0.1097/JES.0b013e31824c43af.
• 5-Week Introductory Heat Training Plan
• Basics of Heat Training
• Passive Heat Training Load Calculator
• Boosting Your Heat Adaptation Score
• Women’s Core Temp Variations
• The Performance Effects of Sauna Use
• The Norwegian Heat Training Method – Final Race Prep
• Individuality of Heat Zones
• Integrating Heat Sessions into Your Training Plan
• Heat, Altitude and Haemoglobin
• Heat Training Load
• Heat Strain Index
• Heat-Related Performance Loss
• Heat Adaptation Score