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The concept of "heart rate" is a medical one, but it's becoming more popular thanks to the popularity of smart wearable devices (smartwatches/wristbands). At present, almost all smartwatches/bracelets have the function of "heart rate" detection and even monitoring. But do you really use a smartwatch/band to detect your "heart rate"?

1. What is "heart rate"?
Heart rate, as the name implies, refers to the frequency of Heart beating, usually represented by the number of Beats Per Minute, namely BPM (Beats Per Minute). Heart rate is one of the basic physiological characteristics of the human body. The resting heart rate of normal adults is generally 60 ~ 100BPM, while the ideal heart rate range of adults is 55 ~ 70BPM. Neither too high nor too low a heart rate is good, but it is impossible to judge a person's true health status from the heart rate alone. It must be combined with other indicators.

2. Does the smartwatch/bracelet measure "heart rate"?
First, the bottom line: smartwatches/bracelets don't actually measure "heart rate."

Currently available smartwatches/wristbands use a method called Photoplethysmograph (PPG) to obtain the "heart rate."

PhotoPlethysmoGraphy (PPG) is a non-invasive method to detect blood volume changes in living tissues by means of photoelectricity. When a certain wavelength of light beam shines on the skin surface of the finger tip, the light beam will be transmitted to the photoelectric receiver by transmission or reflection. During this process, the light intensity detected by the detector will be weakened due to the absorption and attenuation of the skin muscle and blood at the finger end. The absorption of light by skin, muscle and tissue is constant in the whole blood circulation, while the blood volume in the skin shows pulsatile changes under the action of the heart. When the heart contracts, the peripheral blood volume is the largest, the amount of light absorption is the largest, and the detected light intensity is the smallest.

When the heart is diastolic, on the contrary, the detected light intensity is the highest, so that the light intensity received by the light receiver will show a pulsatile change, and the change of the light intensity signal can be converted into an electrical signal to obtain the change of volume pulse blood flow. It can be seen that volume pulse blood flow contains many important physiological information of cardiovascular system, such as cardiac function and blood flow. At the same time, volume pulse blood flow mainly exists in microvessels such as arterioles and capillaries in peripheral blood vessels, so volume pulse blood flow also contains rich physiological and pathological information of microcirculation, which is an important information source for us to study the human systemic circulation system.

No matter more than 2,000 Apple Watches or more than 100 Xiaomi wristbands, they all use this photoelectric sensor based on the photoelectric volume pulse wave principle.