Heart rate variability (HRV) is the physiological phenomenon of variation in the time interval between heartbeats and is measured by the variation in the beat-to-beat interval (commonly stated as R-R interval). It is well known that HRV is directly related to the body's interdependent regulatory systems and can be a powerful measure for an athlete's recovery status.
Metrics for HRV
Root mean square of successive differences. This value is said to be a strong indicator for parasympathetic activity and thus large values indicate a better recovery status.
Natural logarithm of RMSSD. The logarithm is used to achieve a better scaling.
Standard deviation of R-R intervals
Standard deviation of the average R-R intervals calculated over 5 minutes
Proportion of successive R-R intervals that differ by more than 50 ms
Proportion of successive R-R intervals that differ by more than 20 ms