Fitbit Flex Wireless Activity and Sleep Tracker Wristband: Slim, stylish and always by your side. Track your activity throughout the day and monitor the quality of your sleep at night. LED lights on the device show you how your day is measuring up against your personal goals, motivating you to achieve a more active lifestyle.
Does it track biking? Could it be worn on the ankle (maybe under a sock to keep from losing it?)
Yes, you can wear it during biking. Because it even track while driving. you maybe able to wear on your ankle if it very small. Otherwise, you can only wear it on your wrist.