Some science behind the scenes

Electrocardiograph

The electrocardiograph (ECG) uses electrodes placed on the torso, wrists, or legs, to measure the electrical activity of the heart and measures the interbeat interval (distances between successive R-wave peaks in the QRS complex).

The interbeat interval, divided into 60 seconds, determines the heart rate at that moment. The statistical variability of that interbeat interval is called the  heart rate variability.

 The ECG method is more accurate than the PPG method in measuring heart rate variability.

It is worth adding that Biofeedback therapists use HRV biofeedback when treating asthma, COPD, depression, fibromyalgia, heart disease and unexplained abdominal pain.

Observations

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