Cardiio helps you measure your heart rate, and perform workouts to get in shape.

How Cardiio works

Blood absorbs light. Every time your heart beats, the blood flow to your face and finger increases, causing more light to be absorbed. In between beats, less light is absorbed. By using your smartphone camera to capture tiny changes in reflected light from either your finger or face, Cardiio can calculate your heart rate. Cardiio is based on technology licensed from MIT and originally developed at the MIT Media Lab. MIT has not endorsed the Cardiio app or Cardiio, Inc.

Cardiio is one of those apps that wows you the first time you use it, because you never thought your iPhone was capable of these things without expensive accessories.
— Cult of Mac