Fitbit (whether Flex or any other models) doesn't care about HRM's: it can't interact directly with them. You'd want to use an app on your smartphone that supports the HRM and that can connect to your Fitbit account. One such apps is Digifit iCardio: when you log workouts in iCardio, the calorie data derived from your heart rate can be sent to your Fitbit account. This can be handy for activities that Fitbit is not good at tracking.
Can you sync your Fitbit with your Razr HD? If so, it's compatible with BLE (Bluetooth Low Emission) and you could have a look at BLE HRM's, for instance the Polar H7 chest strap or the Mio Alpha watch.
Dominique | Finland
Ionic, Aria, Flyer, TrendWeight | Windows 7, OS X 10.13.5 | Motorola Moto G6 (Android 9), iPad Air (iOS 12.4.4)
Take a look at the Fitbit help site for further assistance and information.
Fitbit Product Experts Alumni are retired members of the Fitbit Product Expert Program. Learn more
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