Heart rate sensors on watches help monitor the heart’s blood flow, along with the amount of heartbeats through the the wearer’s artery. It uses photoplethysmographic green light, or PPG, on the watch is the sensor used to determine the blood flow and heartbeats. The problem is that PPG in watches is a pretty cheap procedure and doesn’t give accurate results for darker skinned folks.
A group of scientists decided to run an experiment to test if the watches have accuracy, by allowing about 22 people to run the experiment. The watches they used are from Apple, Fitbit, Samsung, and Mio ALPHA. After the experiment, the scientists discovered that the heart rate monitors had accurate readings, but were off when it came to energy expenditure.
As noted in the chart above, the heart rate monitor was able to get a somewhat accurate reading, as they did not differ by more than 20 bpm, but when it came to energy expenditure, they reached almost 100 kcals. Due to limitations, most readings are inaccurate, and gave the wrong idea that people are burning more or less calories.