Heart-rate variability: what it is and why you should measure it

HRV measurement can be very useful at determining a triathlete's recovery. We look at what it is, and why several measurements could tell you more

Over shoulder view on Black sport woman using tracker software on smartphone and smartwatch. Modern technology and healthy lifestyle concept.

Measuring heart rate variability (HRV) is extremely useful for an athlete as it’s linked to accumulated physiological stress and fatigue in the body. 


In very simple terms, the more physiologically relaxed (ie recovered) the body is, the greater the HRV; the more physiologically tired the body is, the smaller the HRV.

By measuring your HRV, you can therefore determine how well recovered you are, which then helps you plan or adjust your training accordingly.

One-off vs averaged HRV

Most HRV software packages instruct the user to take a measurement of their HRV each day in order to assess how well recovered they are and also to spot any trends – for example, a fatigue index that increases over two or three days.

However, while this can be very informative, new research suggests that a single measurement of HRV might not be enough to properly reflect how well recovered you are in terms of adaptation to an increased training load, and therefore your likely performance.

In a study published just three months ago, researchers looked at the relationship between actual performance (maximum aerobic speed and 10km running performance) and HRV measurements in runners undergoing a nine-week training programme.

In particular, they wanted to see whether averaging out daily HRV measurements over a seven-day period gave a better reflection of how well adapted the runners were (and therefore how well they performed) than taking the HRV measurement on a single isolated day.

It turned out that the correlation between a single HRV measurement and maximum aerobic speed/10km times was quite poor (ie, it didn’t seem to relate particularly well to the runners’ actual performances).

However, when a series of HRV measurements were taken for a week and averaged out, the correlation with actual running performance was very good.

The same group then carried out another study where they looked at changes in HRV between different phases of training (a normal training period, a high-intensity period and a taper period) in triathletes.

They wanted to see how well HRV measurements mirrored changes in performance, but this time they looked at 1-day, 2-day, 3-day, 4-day, 5-day, 6-day and 7-day HRV averages.

It turned out that provided the days selected within the seven-day period were random (ie not consecutive), the average of three days of HRV measurement gave a pretty good reflection of adaptation/recovery – ie, it wasn’t necessary to take an average of all seven days.

The tips below show how you can use these findings to give you a better idea of how well you are recovered in terms of likely performance in a subsequent training session.

How to use HRV measurements 

  • Measure your HRV daily to monitor fatigue/recovery and to spot trends.
  • To determine how well you’ve adapted to an increased training load, take at least three (non-consecutive) days of HRV measurements from the past seven and work out the average score.
  • Always take your HRV measurement at the same time of day – ideally first thing in the morning.
  • Always keep a record of your training so you can link changes in HRV to your training loads (and adjust if necessary).

Top image credit: Getty Images