So......I thought "it can't be difficult to build an App that uses the published Fitbit API, allowing users to access and download their own data - surely?"
Well it turns out that it's not all that straightforward, but I am making good progress. I already have this working for Heart Rate data (which is what I wanted to see in detail), and I'm working on accessing the other types of data, such as step data, calories burned, etc., etc.
The "Intraday data" that's stored on the device I have (I have a Charge HR Fitbit) is very detailed minute-by-minute data, so can be used to assess recovery rates etc.
Anyone wanting to have a play with this, can access it
Enjoy!
