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What is HRV and how do smart watches measure HRV?


Heart rate variability (HRV) is already in everyone’s mouth, at least a common term for those interested in monitoring their own or their clients’ sleep, recovery, performance or overall health. Many professional and consumer wearable devices list HRV as one of the indicators they measure. Sports, health and performance coaches and professionals should understand the basics of this fascinating phenomenon.

Without delving into the science of HRV, this article will discuss HRV from a practical perspective: what it is, what information it can tell you about your physical physiology, and what you should pay attention to when explaining it.

“Higher HRV has been found to be associated with a reduction in morbidity and mortality, as well as an improvement in mental health and quality of life.”

Heart rate variability or HRV is a physiological phenomenon in which the time interval (in milliseconds) between consecutive heartbeats changes. A normal, healthy heart does not tick evenly like a metronome, on the contrary, it will constantly change when observing the milliseconds between two heartbeats. Generally speaking, we are not keenly aware of this change. It is different from the increase and decrease of heart rate in our daily work.

If you feel the pulse on your wrist when you take a few deep breaths, you can understand your heart rate variability: the interval between heartbeats becomes longer when you exhale (the heart rate slows down), the interval between heartbeats becomes shorter when you exhale (the heart rate increases), inhale, This phenomenon is called respiratory sinus arrhythmia. In addition to breathing, HRV is also severely affected by, for example, exercise, hormonal responses, metabolic processes, cognitive processes, stress, and recovery.

How to measure heart rate variability?

Reliable HRV analysis requires accurate measurement of each heartbeat and the time between heartbeats. There are many techniques for calculating HRV. Nowadays, the daily measurement of heart rate variability is mainly through the HRV function of smart watches. For example, the current HRV function of Huawei Watch 3, Apple Watch and Veepoo Watch RIG can measure heart rate variability very accurately. The working principle of these smart watches is mainly based on the ECG method to detect R waves in the QRS complex and calculate the time between R waves (RR interval; Figure 1). It can detect the heartbeat with an accuracy of 1 ms (1000HZ), and perform very accurate HRV analysis on most people of different body types and age groups. Most widely used wearable devices use PPG or photoplethysmography to optically detect the heartbeat by measuring blood flow waves from the wrist or ear, and then calculate the heartbeat interval or IBI. Comparing different methods is always challenging, which is certainly true for HRV. However, different methods and equipment, if used correctly and systematically, can provide users with interesting and useful information.

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