Garmin Resting HRV - my experience

Garmin Resting HRV - my experience

As the basis for many fitness related concepts, the measurement of resting HRV is of critical importance. These metrics include "readiness to train", sleep quality/stages and autonomic strain/recovery. Consequently, even if some of the newer "hyped" tracking devices have these features, we are still at the mercy of whether they can accurately measure resting HRV (yes I'm talking about the Fenix 8 and the newer "rings"). Over the past several years, I have used numerous Garmin devices, including the Fenix 5, 6, 7, Epix 2 and FR965. Despite generational advances in the optical sensors (according to the manufacturer), my experience has not been positive regarding resting HRV (or exercise HR). Time after time, my H10 or Movesense ECG overnight readings (using Kubios with an ECG) were 200-300% different from Garmin! This frustrated me to no end considering my interest in accurate HRV.....So, I sought a solution to the problem.

 

Let's go through some of my testing and a solution to this issue. 

Baseline - A typical night recording with my Garmin FR 965:

• Garmin FR 965 (wrist) RMSSD = 47 ms:

 

Comparison to Gold standard 

And at the same time wearing a Movesense ECG, sample rate 250 Hz with Kubios HRV results: 

• Movesense ECG with Kubios, full night average RMSSD = 18 ms:

• The Garmin unit is about 2.5x too high!
  
• I've done this many times before on all the Garmin devices mentioned above, and if anything, the error is getting worse with the newer units. You can see my Garmin range of 46 to 56 ms, which I never see on ECG overnight measurements.
• Hence, the Garmin HRV related metrics/recommendations/sleep phases are assumed to be inaccurate. 
  

My solution:

My wife has an Apple Watch and from my testing (as above) the HRV and exercise HR have been spot on! But I'm an android user, so that's out for me. My thought was that perhaps the big "players" like Apple (who could attract the best developers through higher budgets and benefits) might provide the best accuracy, so I did the next best thing - I bought a Pixel 3 watch (Google/Fitbit). 

Here are the initial HRV results (I've only had it a week or so).

Gold standard -  

• Movesense ECG with Kubios, full night average RMSSD = 14.3ms:

 

• Pixel Watch 3 wrist (RMSSD = 14 ms):

• As you can see with the Garmin (46-50ms) and Pixel (14-18ms) daily RMSSD numbers, they are internally consistent within device, with only small changes on a day-to-day basis. 

What surprises me the most is not the huge error in the Garmin metric (I already knew that for years), but how close the Pixel was aligned with Kubios/ECG results. This is very encouraging, and I'll be keeping track over the next few weeks to make sure the good agreement continues.

• What does this mean for those of you using various rings and watches? It means that your device may or may not be in alignment with true resting HRV. If it is, then great, but if not, then the "recommendations" put forth by (let's say) Garmin may be fatally flawed.

The solution

There is a simple way of knowing where you and your device stands - use the Polar H10 chest belt, record a full night of data (with Fatmaxxer, Garmin units or other apps) and use Kubios (free version) to ascertain the RMSSD. You may want to temporarily disable the Garmin from its pairing with the H10 just to be sure that it's using optical input.

In this post I have a brief tutorial on how to use Kubios free version (you will need to use a measurement window size of the entire night, not 2 minutes as used for DFA a1). If this is too complex, there is a simpler way - use Fatmaxxer, which does log the RMSSD to the "features" file. Yet again, another great feature of Fatmaxxer! These would be 2-minute window measurements, so you will need to average the column:

This is the "features" file from Kubios (not mine, a random test case):

 

My night data test: Full night measurement window vs averaging all the 2 minute Fatmaxxer measurements:

• I did a comparison between the RMSSD of an 8 hour nightly session as a single measurement window in Kubios (8 hrs) vs the same RMSSD as 2 minute windows averaged (over 6000 overlapping Kubios samples). The results were quite close - 17 vs 18 ms.

 

Exercise HR - Outdoor road cycling

The other item I was curious about was how well the Pixel 3 watch optical HRM agreed with the H10 in simple HR. In the past, the Garmin devices have been dismal for outdoor cycling and even unreliable indoors on a trainer (for me). Therefore, I did about 3 hours outdoors on bumpy roads with a few intervals. Here is a partial sample of the results:

• You get the picture - and it's impressive for a wrist device during road riding at various intensities.

Why should we care about absolute values?

You may think that it's ok to have this systematic huge bias in HRV as long as the trend is helpful. Perhaps. But there is a great deal of value in knowing your true resting RMSSD (or SDNN). Here are a couple of interesting studies looking at mortality prediction and biologic aging

 

Summary and Thoughts

• The above results are based on an N=1 sample, me. Perhaps your device/wrist combo will function differently and match to a gold standard. However, I do want to caution getting a false sense of confidence after a single good correlation between a Kubios or Fatmaxxer RMSSD sample and your device. What if "X" vendor's RMSSD (Garmin, Suunto, etc.) just coincidently matches your reading with Kubios? After all, my Garmin readings are certainly plausible for many individuals and could match gold standards just from statistical luck. How do we get around this circumstance? In that case, one could follow along for several nights and make sure the correlation holds up.
• Further, this type of discrepancy in a basic wearable metric vs an established method could be present elsewhere in a company's product. If Garmin can't get optical HRV correct, what else are they messing up that we can't easily measure? This illustrates the need for us to "trust but verify". It's unfortunate we can't blindly trust what the various wearable vendors are saying, but these are not FDA approved medical devices (therefore, anything goes). 
• The other clue to optical HRM quality could also be how well the HR tracks during exercise. If your wearable aligns well with a chest belt, especially during intervals, that could be a favorable sign. 
• I have abandoned my Garmin FR965 after being a loyal user for many years. I still use the Edge 1050 for cycling (which is great for recording anything ANT+ or bluetooth and displaying alphaHRV a1). I'd rather have accurate resting HRV (and wrist exercise HR) than the nice watch faces.
  
• Bottom line - Resting HRV is a promising metric for readiness, ANS stress, illness and recovery. Please make sure your readings are trustworthy....