Your Apple Watch is already a pretty decent fitness activity tracker. Thanks to Tennis Plus, you can turn it into a swing analyzer. The app detects your swings in real-time and measures your speed & spin. You will also be able to see time elapsed, calories burned, and heart rate throughout each practice session.
To use this app, you don’t need a separate sensor. Simply put your watch on, and you are ready to go. Tennis Plus can give you a breakdown of your shots (forehand, backhand, topspin, flat, slice, volley, smash, serve).
[where to get it]