Heart rate variability (HRV)

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Heart rate variability, or HRV, is becoming an increasingly popular health metric to track and for good reason.  HRV can indicate the state of your nervous system, track recovery and even be a potential predicter of a future cardiac event.  Simply put, HRV is the time interval in between heartbeats.  While you might think that you want very a consistent heartbeat like a metronome, or low HRV, this is actually an indicator of poor health as the sympathetic "fight or flight" branch of your nervous system increases cardiac output and decreases HRV, which is needed during exercise and mentally stressful situations.  Conversely, a high HRV, much more variation in heartbeats, is associated with lower cardiac output, better ability to manage stress and the parasympathetic or "rest and digest" part of your nervous system.  

I personally use HRV to determine the type and/or intensity of the workouts I do.  For example, if I had planned a longer run for the day, but my morning HRV was below 50ms I may opt to shorten the run.  In the rare event my HRV was below 25ms in the morning I would cancel the run altogether and substitute a short walk or yoga session.  The opposite is also true.  If I had a shorter run planned for the day and my morning HRV was say over 150ms I would either lengthen the run (if time permitted) or just intensify the run and go for a personal best.  

While it is important to understand your own baseline HRV score and to monitor how it trends and changes over time, it is also nice to have population statistics as a comparison of your own baseline.  The chart below has average HRV scores for some of the more popular wearables, as well as plus/minus one standard deviation for different age groups.