Also, you can manually enter your maximum heart-rate if you log in to your fitbit account webpage on a browser. Click on the setting gear icon a the top right. Then Settings->Personal info->Advanced settings->Heart rate zones, and set with your true maximum if you know it. It improves the accuracy a little. Even people with average resting heart rates for their age (not me) can have lower or higher actual maximum heart rates, which seriously affect the "zones" for activity.
