For the few weeks I have been using the FitBit Flex, I was very surprise with my step numbers regarding my bicycling activities.
Now I can report that the FitBit Flex do not track bicycling activity. Even in the sensitive mode.
It was not obvious to find an answer on the internet too, I did find this post
The FitBit Flex Will Get Your Ass Off The Couch, But Not Through A Marathon
which also mention it.
Since I am focusing on low impact activity I do 50% of my daily activity on a bicycle.
I need to find a replacement.