CORE 2 on Apple Watch

If you train or race in heat, heart rate and pace do not tell the whole story. Two runs can look similar from the outside, but feel very different if your body is working harder to stay cool.

That is where the CORE 2 Thermal Sensor fits in. It gives endurance athletes a way to monitor thermal data during training and racing, including core body temperature, skin temperature, and Heat Strain Index.

With Watchletic, you can connect CORE 2 directly to Apple Watch and use that data inside the same workout app you already use for structured workouts, Bluetooth sensors, Apple Health, and exports.

Why use CORE 2 with Apple Watch?

CORE 2 is built for athletes who care about heat.

During hard sessions, hot races, indoor training, or heat adaptation blocks, core temperature can rise even when pace or power looks controlled. Watching that trend can help you make better decisions: hold the effort, cool down, drink, back off, or learn how your body reacts over time.

The official CORE 2 specs are also very practical for endurance sport. It uses Bluetooth Low Energy and ANT+, weighs 8.6 g without the clip or 11.4 g with the clip, is rated IPX7 waterproof, and has about 5 days of battery life, or up to 30 days with standby mode enabled.

CORE recommends using a heart rate monitor for the most accurate sports use, because heart rate helps the sensor estimate core temperature during exercise.

What Watchletic shows from CORE 2

When CORE 2 is connected to Watchletic, you can use these values in your Apple Watch workout layout:

• Core Temp
• Skin Temp
• HSI (Heat Strain Index)

That means you can put thermal data next to the values you already use, such as pace, power, heart rate, distance, interval time, or lap data.

For example, during a hot tempo run you might keep pace and heart rate visible, but also add Core Temp or HSI to see whether heat is becoming the limiting factor.

Watchletic supports both Celsius and Fahrenheit display for temperature values, depending on your temperature unit setting.

What gets recorded

Watchletic stores CORE 2 samples during the workout:

• core body temperature
• skin temperature
• Heat Strain Index

Core body temperature is also written to Apple Health as body temperature during the workout.

Skin temperature and Heat Strain Index are kept in Watchletic's workout data, shown in the workout details graph, and included as part of exported FIT files.

Workout details and graphs

After the workout, Watchletic can show the CORE data in the workout graph:

• Core body temperature
• Skin temperature
• Heat strain index

This is useful because the important part is often the trend, not just one number.

You can look back and see whether your core temperature climbed gradually, whether it stabilized, or whether Heat Strain Index rose during a certain interval, climb, indoor block, or race section.

FIT exports

Watchletic includes CORE data in FIT exports.

Core body temperature is written as the standard FIT core_temperature field. Skin temperature and Heat Strain Index are written as FIT developer fields:

• skin_temperature in C
• heat_strain_index

That makes the completed workout more useful in analysis tools that understand those fields, while still keeping the workout recorded from Apple Watch.

What about heart rate?

CORE 2 itself requires heart rate input for best sports accuracy. Depending on your setup, CORE can retransmit heart rate data from a paired monitor.

Watchletic handles that carefully. If CORE is emitting heart rate and you do not already have a separate heart rate monitor connected to Watchletic, Watchletic can use the retransmitted heart rate as the workout heart rate source.

If you do connect a dedicated Bluetooth heart rate monitor directly to Watchletic, that monitor remains the heart rate source. This avoids mixing multiple heart rate sources in the same workout.

How to connect CORE 2

To use CORE 2 in Watchletic:

1. Wear CORE 2 according to CORE's instructions.
2. Open Watchletic on Apple Watch.
3. Go to Settings > Bluetooth.
4. Choose the workout type you want to use.
5. Tap Search.
6. Select your CORE 2 sensor.
7. Start your workout from Watchletic.

Once connected, add Core Temp, Skin Temp, or HSI to your Watchletic workout layout if you want those values visible during the session.

Final words

CORE 2 is not just another number on the watch. For heat training, hot races, indoor treadmill work, or long summer sessions, thermal data can explain why the same pace or power feels different from one day to the next.

With Watchletic, Apple Watch runners can use CORE 2 as part of a broader workout setup: structured sessions, live Apple Watch layouts, Bluetooth devices, Apple Health recording, workout graphs, and FIT exports in one place.