Heart Rate Notifications
How can I set a High Heart rate notification on my Apple 5 watch that is lower tan 100 bpm?
Apple Watch Series 5, watchOS 6
You can't, 100 is the lowest option available. There doesn't appear to be a way to customize the rate.
ASC - Late 2012 Mac mini, macOS 10.15.2; Watch, series 5 GPS/cellular, watchOS 6.1.1; iPad Air 2, ipadOS 13.3; iPhone 8+, iOS 13.3; TV 4th Gen, tvOS 12.4.1; HomePods, audioOS 13.3 and Airport Express w/ Airplay 2