How to track your heart rate with only your smartphone - CNET
Optical vs chest strap heart rate monitors - Measuring beats per minute using different sensors
How to track your heart rate with only your smartphone - CNET
Heart rate monitor - Wikipedia
Cardiac Event Monitor: Types and Uses
Are there heart rate monitors which can be used without a watch or phone (i.e., record data on the HRM to be later synced to a phone after the workout)? - Quora