Above: Jawbone's Up health wristbandImage Credit: Devindra Hardawar/VentureBeatJawbone’s $150 Up24 may be the best fitness tracker of 2014. But the “fitness tracker” category of wearables is getting crowded, and Jawbone’s competitors are bringing new features into the category that fitness-minded consumers will surely buy. Chief among those features is the heartbeat sensor, which can work in concert with the accelerometer and the app to track things like calories burned.