Heart Rate Variability

[UPDATE 12/30/2015] While going over the additional data, I realized that I had a typo in Excel formula while computing RMSSD (Root Mean Square of Successive Differences). As a result, instead of average of all RR intervals, I was using average of only the first and last RR intervals. Oops. So I updated the RMSSD numbers and my concluding comment.


One of the features of Polar V800 is RR Recording. It measures heart rate variability (HRV). Heart rate variability is the differences in consecutive heart rates. For example, if heart beats at t=0, 1, 1.9, and 2.85 second, the heart rate would be 60, 66.7, and 63.2 beats per minute, and the HRV would be 6.7 and -3.5 bpm.

It’s also often expressed as RR-intervals, which is short for R-wave-to-R-wave interval. R-wave is the wave created by heart beat where the peak is found. RR-interval is also known as Inter-beat Interval (IBI). In the above example, RR-intervals (or IBIs) would be -0.1 and 0.05 second. (For more information on R-wave, visit here.)

It is nothing but a calculated value from straight off heart beat measurement. So what’s the deal? Why would anyone bother measuring their HRV?

It turns out that the HRV has been used in psychology experiments to represent how relaxed the subjects are, and how ready they are to deal with external stresses. The greater the HRV is (meaning the greater the heart rate varies from second to second), the more relaxed the subject is. On the other hand, the smaller the HRV is (i.e. the heart rate is more or less constant), the more stressed she is.

If anyone cares to recall their human anatomy class, HRV measures whether sympathetic or parasympathetic nervous system is kicking in. Sympathetic nervous system has to do with fight-or-flight response. It increases heart rate, and dilates pupil among other stress-related physical responses. Parasympathetic nervous system has to do with slowing down physical responses, such as decreased heart rate, stimulating digestive system, etc.

Because the HRV provides a simple measurement of how relaxed and ready we are, some athletes are using it to see whether they are over training. Thinking is that if the HRV shows little variation, we are under stress, and may not be the right time to start the training.

When I found out about HRV, it seemed counter intuitive. I would have thought that if I’m relaxed, I would see little variation on my heart rate.

Another thing that seemed odd to me was that I couldn’t find a scale (even relative scale) of what typical HRV range would be.

So I decided to experiment.

Minutes before my 15-mile run yesterday, I took my HRV measurement to establish my baseline. Not knowing what to expect, I wanted to get a good sample size, and I figured 5+ minutes would have enough sample points (about 300+ heart beats). Also to minimize the effect of other stresses, I took the measurements while lying down.

Here’s what my RR intervals looked like before my run.


And here’s what they looked like an hour and 45 minutes after my 15-mile run.



As predicted by literature, my heart rate varied noticeably more before my long run, compared to the one after my long run.

To quantify the variability, I ran the Root Mean Square of Successive Differences (RMSSD) of the 337 heart beats. (When I was rested, I had about 337 heart beats in 5 minutes.) BTW, if you want to find out about how to compute RMSSD, you can reference this article that I found.

And to measure how fast I was recovering, I took two additional 5-minute measurements, 23 hours and 45 hours after the run, and computed the RMSSD.

  • RMSSD 00:25 before 15-mile run: 25.551 msec
  • RMSSD 01:45 after 15-mile run   :    4.505 msec
  • RMSSD 23:00 after 15-mile run   : 14.396 msec
  • RMSSD 45:00 after 15-mile run   : 42.032 msec

It looks like my HRV drops to about 18% after my long run. Surprisingly I seem to get full recovery of HRV (plus some more!) within 45 hours of the long run!

That’s way faster than what my V800 estimates as my long run recovery time.

Oh well, even if my heart may be willing, my sore feet say I still needed a rest today. 🙂


5 thoughts on “Heart Rate Variability”

  1. Love seeing HRV being tracked multiple times throughout the day. It adds a whole new dimension to tracking recovery. Also great point that sore muscles/feet can be just as good of a reason to rest as systemic stress load! Great write up – cheers

    1. Hi Jason,

      Thank you for stopping by, and adding to the conversation. Listening to what our body tells us is something that I begin to appreciate even more after taking some measurements. Happy Holidays.


      1. Nice article. Did you know you can autmaticly show the rMSSD in polar flow by selecting a range in the rr interval reading?

        My rMSSD is always well below 25, more like 21,22. Maybe i really need to take a week off. Or I need to measure more RR intervals and more Orthostatische tests to get to know when I’m overtrained…


        1. Hi Stephan,

          Thanks for stopping by, and providing a tip. I did not know Polar Flow website displayed RMSSD. I will check it out.

          As for RMSSD scale, I have a feeling that it varies by individual. For me, it looked like 10 – 24 msec is the optimal training range. One thing I learned was that, it’s important to take HRV measurements while completely relaxed lying down.

          Happy training!


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