Making sense of heart rate variability (HRV) score

[AxB]

I have read many complaints and concerns related to the HRV values given by Fitbit devices. In the sense that they appear too low for what one would expect or much lower than values given by other measurements. Which made a lot of people worried about their health condition or simply discard Fitbit values as nonsense.

I am not an expert on this field or anything like that, but these are a few pieces of information I gathered about this issue since I was also wondering about it.

On the Google Fitbit Help Center website as well as within the Fitbit app one can read the following:

'Fitbit uses the common formula called RMSSD to determine HRV from your recent heart-rate data.' 

Based on other sources, it appears that there are many methods to calculate and analyze HRV, including RMSSD. So, what Google states in the above quote seems to make sense. They are calculating the HRV based on one of the formulas available for this purpose.

Further, there seems to be two main formulas used to calculate HRV, and again, one is the RMSSD used by Fitbit and the other is called SDNN. The main thing is that these two formulas  produce different values for the same condition. So, if you use a measurement based on the SDNN formula, the results will be higher than those measurements relying on the RMSSD formula.

The RMSS formula is quoted to be a standard statistical measure of HRV and it basically refers to Root Means Square of Successive Differences, therefore RMSSD. It measures the time difference between each successive heartbeat in real-time to arrive at your RMSSD score. This formula seems to be helpful for investigating the impact of training loads and recovery processes.

And now the most important part.

It seems like based on the review of different studies related to normal values for short-term heart rate variability in healthy adults, the following HRV values are supposed to be normal:

Measurements based on SDNN (ms) 32–93

Measurements based on RMSSD (ms) 19–75 (what Fitbit uses)

To put all this into some perspective. In my case, based on the SDNN formula, I am getting values from 40 to 66 ms. My HRV scores on Fitbit are between 30 and 39 ms, which indeed fits under the RMSSD formula values above.

I hope this helps.

[RiekoC]

Hello @AxB 

Thank you for taking the time to explain how Fitbit calculates HRV as well as the normal range. This will be very helpful for those concerned with their Fitibit HRV data.