How do heart rate monitors work?

1 Answer
Jul 11, 2016

The two different types of heart rate monitors work in different ways.


Chest strap monitors

A chest strap monitor consists of a transmitter attached to a belt worn around the chest and a receiver worn on the wrist like a watch.

When your heart beats, a small electrical signal is sent through the heart muscles, causing it to contract.

The monitor detects this electrical signal through the skin and transmits it via Bluetooth to the wrist receiver, which displays the heart rate.

Wrist watch monitors

Wrist monitor

Wrist watch monitors use optical sensing technology called photoplethysmography (PPG) (Greek photos = "light" + plethysmos = "enlargement"+ graphein = "to write"), that is, using light to measure volume changes in the blood vessels.

Blood absorbs green light, so the back of the watch contains green LEDs that flash hundreds of times per second.

Each heart beat brings a spike in blood flow and changes in green light absorption.

Photodiodes measure the changes in light intensity and display the information as a heart rate on the front of the watch.