rubyfitkm is my fork of & modifications to Cullen King's rubyfit gem.

The following is more-or-less verbatim from Cullen's document:

rubyfit is here to help your sanity when working with FIT files.

This project contains the FIT SDK version 10.20, so pay your respects: This is Ant

Anyone familiar with stream based XML processing should find using this library intuitive.

1. Create a callbacks class that responds to the callback events, and handle the events as needed. Use the example fit_callbacks.rb file as a guide - Our callbacks class on ridewithgps is more complex, as it needs states to properly process laps, etc.
2. Pass in your callback class instance to the RubyFitKM::FitParser initializer.
3. Call parse() on your parser instance.

Example:

raw = IO.read("myfitfile.fit")
callbacks = FitCallbacks.new()
parser = RubyFitKM::FitParser.new(callbacks)
parser.parse(raw)
activities = callbacks.activities #assumes you have some sort of getter/attr_reader on your custom callbacks class

When I get more time I'll document the messages, but for now you can look in ext/rubyfitkm/rubyfitkm.c to see what fields are being passed.