Practically speaking, our heart does not beat at a constant frequency. So even if we measure our pulse and get a 60 beats per minute (bpm) reading, it doesn’t mean we have a beat every second. The time differences between beats are slightly different, they can be 0.9 seconds, 1.2 seconds etc. The Heart Rate Variability indicates the beat to beat (R-R interval) variations.
The following figure illustrates an example of the time variation between R-R intervals.
HRV is significantly impacted by aerobic fitness. The HRV of a well-conditioned heart is typically large at rest. Other factors that affect HRV are age, genetics, body position, time of day or health status. During exercise, HRV decreases as heart rate and exercise intensity increase. HRV also decreases during periods of mental stress.
There are a variety of different HRV metrics. One well established metric -- used here -- is the root mean squared of successive differences (RMSSD) of heart pulses, measured in milliseconds. Studies of healthy adult populations have found measured a mean value of 42 milliseconds within a broad range of 19-75 milliseconds. Young, highly trained athletes may have values up to 120 milliseconds.