Changes

The time between heartbeats varies slightly, even when the average heart rate [[Heart Rate]] is constant. For example, a heart rate [[Heart Rate]] of 60 BPM is an average of one beat per second. However the actual time between heartbeats could vary so that some beats occur after 0.8 seconds, and some after 1.2 seconds. Generally, heart rate variability is greatest at rest and the variability declines as the heart rate rises. Heart rate variability is linked to aerobic fitness, with the fittest individuals having the greatest variability, and this can be used to predict [[VO2max|V̇O₂max]]<ref name="HRV"/>. Some [[Best Running Watch| Running Watches]] can display heart rate variability.

<ref name="HRV">[Heart rate variability and physical exercise. Current ... [Herz. 2006] - PubMed - NCBI http://www.ncbi.nlm.nih.gov/pubmed/17036185 </ref>