And we will cut to the chase and say that the heart rate sensor actually is pretty accurate (close to that of the Apple Watch), although the temperature always seems to be a little on the higher side. You will need to use Sennheiser's companion app for it, though. And if you want to share the data with other fitness apps like Strava, you can do so, although you will need to sign up for the Polar Flow application (it's free, relax). You will also need the application if you want trends of heart rate or temperature as the app by default will just show you. a single figure for each,