I'm an author of a free and open source mobile app Track My Indoor Workout (https://trackmyindoorworkout.github.io/) which records workouts with detailed time series containing non cumulative metrics speed / pace, power (W), cadence, heart rate, GPS location and cumulative metrics elapsed time, distance and burned calories. The app's primary focus are indoor workouts on Bluetooth Smart fitness machines, and one speciality is that TMIW uses the athlete's speed / pace to generate a GPS track as if the athlete would circling laps around a track. This is along the same line if someone would ride along a GPS route in London with Zwift or Sufferfest, just I'm simplifying it to a specific track.
I'm already able to export in FIT and TCX formats. I'm able to upload to Strava, SUUNTO (also a wearable manufacturer as FitBit), MapMyFitness / MapMyRide / MapMyRun / Under Armour, and Training Peaks. Most of the portals require FIT file, whereas UA requires a custom JSON format time series data. I can generate any kind of JSON data FitBit will require (if needed), however I cannot find an API endpoint which I could use.
I really hope that FitBit is open for upload such as SUUNTO.
