For what it's worth, I loathe this sort of thing (damning by vague questioning): "But, if a composite is full of a measure that’s biased, how accurate is it going to be?"

Well, I don't know, and you haven't told me. Is it a consistent high bias? Low? Variable? How biased? How accurate?

What you're trying to say, without accepting responsibility for actually saying it, is "It's an inaccurate composite because it's based on biased measurements." But you don't know that, or you'd say it without weasel words.

> Researchers found the method Apple uses, standard deviation of normal-to-normal, or SDNN, is the most accurate. Other brands implement a system called root mean square standard deviation, RMSSD, which Tenan said produces a wider range of error in the measurement

This is interesting for a couple of reasons. Firstly, I have one so it’s cool to know Apple designed it well in that respect, and that I shouldn’t lose confidence in the readings I get.

The second reason it’s interesting is that this kind of decision, which time-domain measure of heart rate variability metric to use, is the kind of super small design decision that really makes a system special.

Having really good developers and designers really shines through for moments like these.

This is why I use a chest strap heart rate monitor as a cyclist. I see the difference between when my hands are down on the handlebar. It varies around 10-15 bpm (the watch is lower than the chest strap in this scenario). Also, while hiking I see the difference after 2-3 hours. Hikers know the hands swell after 2-3 hours of hiking (you will see people raising both hands while running, that's normal) and this affects my watch heart rate calculations. There is always a discrepancy between the strap and wristwatch.

Watch tracks if my heart rate is high or not but for intense sports like mountain biking I don't expect it to be accurate at all.

I am using a Garmin Instinct 2X Solar Tactical (watch) and a Garmin heart rate chest strap.

"But, if a composite is full of a measure that’s biased, how accurate is it going to be?"

Does it matter if the measure is biased, if you're measuring over time and looking for comparative differences?

I've been tracking HRV for about 8 years and can conclude that (for me) any HRV measurement overnight using a wearable or related wearable readiness metrics are largely useless. Now, what I do - as recommended by Marco Altini, an HRV researcher - is measure in the morning, steated with a chest strap. I use his app, HRV4Training which I would recommend [1]. It's the only way to get decent data in my experience. HRV is so senstive to various stimulii you need to measure it under exacting conditions, which you don't really get when you are sleeping. Sleep stages matter, how you are breathing, body temperature, etc.

I think my Oura ring is great but... it consistently records average nighttime HRV of around 15ms, which is implausable. However, when I wake up and measure with my Polar chest strap I'll get a more sensible reading. Fully recovered it will by about 150ms. Hurting after a hard workout it will be around 40ms. Some other interesting experiences with oura/garmin/apple are:

1. Higher nightime HRV after drinking alcohol than abstaining for weeks but then I measure with a strap and it will be low as expected

2. HRV spikes when I get out of bed in the middle to the night to do childcare related things, when it should be lower when walking around

Another thing to note is that I think I have parasympathetic saturation which means due to physiology and environmental factors you actually end up with a super low HRV when you are lying down/resting. Endurance athletes often get it - I run about 100km a week and my resting HR when fully recovered is about 40 bpm, so I guess that's the reason. When I de-train then nighttime HRV goes back to normal.

If anyone is interested in HRV I definitely recommend reading Marco's blog [1], you will learn alot.

[1] https://www.hrv4training.com

[2] https://substack.com/@marcoaltini

There’s nothing new here, most athletes are aware of these issues, and there are plenty of discussions on Reddit. For instance, you can check out the Garmin subreddit. I've even had problems with fitness bands like Polar.

My recommendation is to be cautious with extreme inaccuracies and interpretations, especially for health-related reasons. It's also helpful to develop greater awareness of your body as part of your training.

Note that the article states that they created a simulation to test. It's unclear how they validated that simulation from the abstract.

This is a weird post. I'm by no means an expert but I've read some papers on HRV and this is extremely well known in HRV research. So much so that these facts (and others) are basically copypasta at the beginning of a lot of papers. People have definitely quantified this before, like a lot. Also, the general advice is that it isn't expected to be "accurate" and the measurements are relative anyway (both to the individual and to the standard of measurement.)

There weren't a bunch of researchers (or even biohackers) walking around who thought SDNN or RMSSD could be substituted for EKG.

Sure, the statistical method matters, but I think it's missing the forest for the tree.

Any metric will differ: bpm, sleep phases, body temp, etc.

Because those devices are not super accurate, and the context you wear them in doesn't allow them to be anyway.

If you were to do a respiratory polygraph with proper medical equipment you'll find different values from your Apple Watch or Oura ring for the times you wake up. But you'll also notice a gap between getting the values at home and in a hospital, because in the hospital the nurses will check your equipment setup.

Those devices are great to give you a big picture, follow general trends and understand your own body. That's it.

I find it incredibly puzzling why Polar is left out of these conversations when it comes to wearable health tech when Polar pioneered a lot of the technology and releases white papers for all their functions.

For example, Garmin has "body battery" which is a pseudo science measure of who knows what and Polar does orthostatic tests (based on HRV). Polar won't even let you do that test with a watch, you need a chest strap because of a lot of similar reasons outlined in this paper

You cannot get HRV from an optical sensor accurately.

Not even at idle, but definitely useless in motion.

And definitely not at the wrist.

Optical at the wrist is all manipulated data via the watch software that is guessing what it is seeing or not.

The only real way is via a heartrate strap that is electrical.

And the next generation of sensors is going to be even more amazing with EKG

https://www.movesense.com/movesense-flash/

Seems odd to compare a machine like a 12 lead EKG to a smartwatch with a single lead EKG support.

Of course variability will differ, that’s why they invest to much into improving the technology and ml algorithms to make up for the difference to personalize the data.

The watch could encourage you to go to a doctor and the doctor can use a more accurate instrument to diagnose.

If you do Apple’s mindfulness breathing exercise it always takes a reading after that which tends to be closer to an accurate HRV.

O’rly????

Every owner of Garmin I know complains of the absolute useless POS it is to rely on the optical HR for HRV. A HRM on a strap will get you good/very good readings for HRV. Finally there is a good podcast from Attia which he talks with an HRV expert behind an HRV dedicated product.

I eventually stopped wearing my Coros running watch because I started to not trust the heart rate thing. Whenever it got cold, the readings shot up! Now I'm just trying to train myself to remember the right tempo from touch.

we had patients come in complaining their smart watch had high readings. after testing they usually had something unusual or indicative. but they were also large... or had other signs.

running an EKG is cheap and easy (might not be cheap to pay for, but it's not an MRI!). i still like patients having more access to health data - legibly - for them. if they were doctors they'd see signs by skin health, etc. the watchs are overall a net positive imo.

.mris are cheap too but a headache to schedule cause everyone needs the machine and there might only be 1 or 2 in a hospital.

I don't like using smartwatches for HR tracking during exercise. If for any reason blood flow to your wrist decreases it becomes inaccurate

you have to measure during periods of deep sleep to get accurate results

i have wildly high HRV and my cardiologist says it doesn't really mean anything

815 ms average HRV when normal is like 50-200 ms

So can we just have our wearables give us both calculations? I don't mind breaking HRV into two measurements, SDNN and RMSSD.

Heart rate variability is just normal sinus arrhythmia that varies with inspiration and expiration. The relative variation is higher with a lower heart rate… I don’t think there is any independent information in it aside from lower heart rate in endurance athletes…