What is it? The Mio Fuse is an activity tracker and heart rate monitor. It is a pretty nifty little device. As an activity tracker, you can set goals, monitor your heart rate, and keep track of steps, calories and distance. It will show you, via the app, how many steps you have taken, and how many you need to reach your goal. The heart rate monitor is an optical sensor. Meaning it shines a little light on your skin and the sensor pick up heart rate.