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HOW DO FITNESS TRACKERS WORK? 

Fitness trackers such as Fitbits or smart watches monitor heart rate using a technique called photoplethysmography.

The tracker sends green light through the skin which is partially absorbed by arteries.

As you exercise, these arteries expand as blood flow increases - meaning more green light is absorbed rather than reflected back to the tracker.

The tracker estimates your heart rate by seeing how much light is reflected back. 

The amount of light that passes back through the skin to the tracker can be affected by the amount of melanin in the skin, and any tattoos.

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