apple watch fitness app closing

Hi

The following steps may help:

Check that Wrist Detection is turned on (on your iPhone, in the Watch app, go to: My Watch (tab) > General > Wrist Detection.

Check that you are wearing your watch sufficiently snugly when exercising:

• Apple Watch should be worn snugly, but comfortably on the top of your wrist.
• Tracking can be affected if your watch loses contact with your skin, causing it to believe that it has been removed from your wrist.
• A loose fitting band can trigger this by allowing your watch to lift from your wrist and/or by allowing clothing (for example, the end of a sleeve) to slip beneath the back of the watch.
• Apple suggests that you consider tightening your band before workouts and loosening it again afterwards.
• Better contact with your wrist may also improve heart rate sensor performance during your workout.

More information:

Use the Workout app on your Apple Watch - Apple Support

Your heart rate. What it means, and where on Apple Watch you’ll find it. - Apple Support

The following further steps may help:

Restart both your iPhone and your watch:

• Close all open apps on your iPhone:
  • Double-click the Home button, then swipe up on each app preview to close it.
• Turn both devices off together, then restart your iPhone first:
  • Restart your iPhone, iPad, or iPod touch - Apple Support
  • Restart your Apple Watch - Apple Support

Unpair and re-pair your watch:

• The Watch app on your iPhone takes a backup of your watch automatically when unpairing via the app. Choose to Restore from Backup during set up. Most data and settings will be restored, with some exceptions (eg Apple Pay cards, passcode).
• Unpair your Apple Watch and iPhone - Apple Support
• Set up your Apple Watch - Apple Support

You're welcome! 🙂