I had a nephew who got excited when he saw HR done on watches. We did some research into accuracy and were surprised to find that the light-based tech used in many devices does not give you accuracy when you are running or generally moving fast. No idea about the fitbit devices but I am vary.

I assumed (incorrectly) that you just needed skin contact measure an electrical pulse (like an EKG machine).

However, that appears not to be the case. Even the wrist based optical ones are often times inaccurate.