Garmin púlsmælir – HRM 600 – M/XL

This heart rate monitor sends real-time data via ANT+® or Bluetooth®. It communicates with compatible Garmin devices or third-party apps like Tacx Training.
The heart rate monitor has built-in running dynamics that include vertical oscillation, ground contact time, balance, and stride length (requires a compatible smartwatch).
HRM 600 measures daily movement such as steps, heart rate, calories, and intensity minutes. All of this is sent by the monitor to Garmin Connect.
If you are running indoors on a track or treadmill, the HRM 600 sends pace and distance to a compatible smartwatch². In swimming, it stores heart rate measurements and sends them to a compatible smartwatch when the swim workout is saved.
The HRM 600 has an LED status light that lets you know when it's on or when the battery is low. The button makes it easy to pair the monitor. It also has 5 ATM water resistance.

You can see which watches support HRM 600 here
https://support.garmin.com/en-US/?faq=0E1YeQaTMi7mouMk5XT5w6