What's My Heart Rate uses the same basic principle, but you can do it without sweating up the phone. It uses the front-facing camera to look at your face for micro-color changes as your heart beats.
In my testing, the app was usually within two BPM of what I was measuring on a watch, which probably means it is right on the money, or more than accurate enough if you're just trying to find out your heart rate on a treadmill, stationary bike, etc.
Unfortunately this app requires perfect lighting conditions and does not interface with apps such as Runtastic, but then again keeping a camera pointed at your face for an entire exercise session is probably not something that would work too well for most.
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