Heart rate variation, HRV
Heart rate varies from heartbeat to heartbeat. Heart rate variability (HRV) is the variation in times between successive heartbeats i.e. R-R intervals, see figure 1.
Figure 1. R-R intervals (s)
HRV indicates the fluctuations of heart rate around an average heart rate. Even though the average heart rate is e.g. 60 beats per minute (bpm), this does not mean that the times between successive heartbeats would be exactly 1.0 sec, but the times can fluctuate/vary from 0.5 sec to even 2.0 sec.
HRV is affected by aerobic fitness. HRV of a well-conditioned heart is generally large at rest. It decreases under physical or mental stress. Other factors that affect HRV are: age, genetics, body position, time of the day and health status. During exercise HRV decreases as the heart rate and exercise intensity increase.
HRV is regulated by autonomic nervous system. Parasympathetic activity decreases heart rate and increases HRV whereas sympathetic activity increases heart rate and decreases HRV.
HRV can be analyzed with linear (time domain method, frequency domain method) and non-linear methods (e.g. Poincaré plot analysis, fractal analysis, entropy analysis).
Polar technology uses HRV measurement and analysis in: Polar SW, OwnOptimizer, EnergyPointer, OwnZone, OwnIndex and OwnIndexs, Relaxation parameter (RLX) and HRmax-p.