https://techcrunch.com/2017/09/12/apple-watch-os4-software/

In Apple’s big product show today in Cupertino at the new Steve Jobs Theater, Apple COO Jeff Williams unveiled Watch OS 4, which is largely focused on health and fitness. The new OS features a redesigned workout app, high intensity interval training, enhanced swim tracking smart activity coaching, GymKit to pair with machines at the gym, a revamped heart rate monitor and new music experience.

With Watch OS 4, you’ll be able to see your heart rate right on your watch face. And when you launch the app, you’ll see your resting heart rate and recovery heart rate, which tells you how quickly your heart rate drops after a workout.