It has become rather trendy to put an optical heart rate sensor into anything even approaching a fitness tracker. Loads of Android Wear watches have them, as does the Apple Watch.
The problem is that not many of them work as soon as you start moving. They're fine, as long as you're not actually 'doing any fitness'.
To find out why, we only need to look at how they work. Optical heart rate sensors (usually) fire green LEDs into your wrist, and then use a tiny little light sensor to measure the level of light. Increased blood flow causes increased absorption of the green light, letting the monitor tell when your heart beats. In theory.
In practice, as soon as you start moving the reliability of the thing goes out of the window because the sensors are micro-jiggled about, meaning the area they're looking onto changes. The Apple Watch seems to combat this pretty effectively by using what appear to be mini lenses to enlarge the sensor's read area – but it doesn't like reading heart rate when you're moving, which is a bit of an issue.
Mio's wrist-worn HR trackers are generally okay, and you can check out the Mio Fuse or TomTom's watches if you're interested. However, the accuracy pales in comparison to a chest strap, and we want to see the tech improve before it becomes ubiquitous.