Flex is powered by your Apple Watch
CooSpo Heart Rate Monitor
Flex has partnered with CooSpo to deliver a real-time heart rate monitor perfect for bodyweight workouts. The CooSpo H8 comes fully equipment with Bluetooth 4.0 support at a 10 meter range. Expect 2-4 day delivery.
Flex supports all mainstream smart watches
Make sure the Flex app downloads on your apple watch before you start and you're good to go!
Unfortunatly Fitbit's cannot be used to collected real-time heart rate and rep data. Fitbit's are designed to be worn throughout the day and, as such, only transmit data periodically to preserve battery life.
Apple Watch Series 1,2,3 & Sport
Pebble Watch (yes they still exist)
If your wearable device is not in this list, don't worry, it could still be supported if it supports realtime data transmission. This is just a list of known wearables.