The heart rate is not completely regular. Heart rate variability (HRV) refers to the time variation between successive heartbeats measured in milliseconds (ms). For example, in rest, it is normal for a person to have an increase in heart rate during inhalation and a decrease during exhalation. The R-peaks seen on the ECG (electrocardiogram) describe the contraction of the left ventricle of the heart, and the interval between successive R-peaks is called the heart rate interval (R-R interval). A high heart rate variability is a sign of a healthy heart.
About one-third of individual heart rate variability is explained by heredity, one-third is determined by physical condition, and one-third is explained by external and internal load factors. Heart rate variability decreases with age, which is automatically taken into account in Firstbeat's reference values.
If you need any further help with this, don't hesitate to contact Firstbeat Support.