You do not get raw data from the fitness trackers.
Here is an article about it.
In short it says:
Fitbit: No access to minute-by-minute (intraday) data with public API,
only with Partner API (needs approval by fitbit)
MisFit: No access to minute-by-minute data (intraday) with API.
There’s only a summary for each day available.
Jawbone: The Jawbone API provides a nice endpoint called ‚moves‘ for
hourly data as well as intervals every minute or even 10 seconds (for
defined workouts)
Withings: No access to minute-by-minute data without activation
through Withings (request needed)
Garmin: Getting detailed information about the Garmin API is quite
difficult. It seems that the only option to get access to tracker
data, is to get access to all data from end users worldwide ($5,000
One-time License Fee).
Sony: Sony only grants access to tracking data through the Lifelog
API. You can therefore read all of the extra lifestyle data you logged
besides the movement data. Could get bulky, but you got the
possibility, if you like. You can recall the tracked activities via
API, subdivided in minute-by-minute data.
You could try to find a device which supports the FitApi from Google (Android Wear device). It supports access to raw sensor data (Example).