I would love to know from somebody with an Apple watch how you use HRV.

I wear a Garmin, which I mostly like. The HRV measurement is predictive enough to be useful. Two occasions from the past 18 months are worth noting:

1) The measure dropped from green through yellow and into red. It was in yellow 7 days. On the first day it was in red, I was hit with covid symptoms. It stayed in red 2 1/2 weeks before starting to climb again; my symptoms had mostly cleared by the end of the first week, but I still felt sluggish for some time after. The kicker is that I hadn't looked at the HRV at all until sometime during the week of symptoms.

2) I went on a trip to a location +8 hours off my usual timezone, and jet lag symptoms were brutal. HRV dropped into yellow on day one of the trip, and stayed there during the 2 weeks of travel. A couple days after I returned it dropped into red, exactly coinciding with the onset of flu/covid symptoms. Again, it stayed in red throughout the illness, and then as I started to get fit again on the other side it ramped up through yellow and into green.

So even with 20% accuracy (if that is the case), it's a useful data point! What does 90% accuracy give you?

About two weeks after I started wearing my wife’s old Apple Watch, my average heart rate jumped up by ~10bpm about a week before I became sick enough to test myself for Covid, which I did indeed have!

Subsequent to getting through the symptoms my heart rate went back down to normal.

Usually people will use it to see if they recover well. If your recovery is bad HRV tends to dip.

btw. in health snapshot Garmin will report both sdnn and rmssd.

Exactly my experience. After my last couple races or long workouts, my HRV dipped a few ms for a few days before coming back to normal. In this case, it’s not telling me anything I don’t already know. But in other instances, a lower HRV would make me consider slowing down a bit.

I have an Apple Watch. I don’t pay attention to my HRV on a daily basis, but if I look at it I can often see in retrospect—for example I had a cold at the beginning of the month which clearly shows up as a ~30ms drop in HRV.

Maybe I should pay more attention to it.

Adding in my anecdata:

I've been tracking HRV since 2018 (I wear an Apple Watch 24/7, aside from charging).

Over 6 years between 2018-2024, I went from being 70 lbs overweight / very out of shape, I then lost all of the weight in 12 months, started going to the gym, became physically fit, and I continue to exercise by cycling 3-4 days a week (sustained heart rate 165 for at least an hour).

Despite going from basically an overweight couch potato to someone very physically fit and strong cardio system from cycling, there has been virtually zero noticeable trend with HRV.

On the flip side, I've noticed resting heart rate correlates extremely closely with cardio fitness and stress. Resting heart rate tends to go up in stressful months or months I don't workout as much. HRV seems to just always be a flat line.

I'm also very skeptical of Vo2 max. For some reason, the watch only records maybe 2 or 3 data points per month at most. Sometimes it doesn't record anything for an entire month even though I'm doing high intensity cardio weekly. Apple rates my Vo2 max as "below average" and similar to HRV, it's a flat line. Seems to not be correlated with my perceived fitness level.

I haven't noticed any correlations with being sick. My impression of all of these metrics (including resting heart rate) is they're maybe useful, but only in the long-term to see trends over time. They aren't very useful for predicting short-term health events, in my experience.

VO2 max is only measured when you do a “brisk walk, hike or run outdoors” according to the Health app. So if you’re doing any other workout type it won’t measure this metric.

Just because it's the most "accurate" doesn'mean it's the most "useful", "correct", or "reliable" in this context. It just means reproducible, if I read it correctly.

In the same way that if they mistakenly chose to represent HRV by the heart rate, this measure would probably be very "accurate" (but ultimately false).

The issue of which HRV measure is the most useful, and what they show exactly, is still an open issue in HRV research afaiu

That sounds useful ! Apple have the best sensors and algorithms, but they are terrible at presenting your data.

Is this true? Do you have a source? I thought that the pros tended to make use of polar’s technology and that their heart rate monitoring (via strap) was considered the most accurate / baseline. (My sources are from what I remember dcrainmaker saying who I consider a well researched reviewer)

Pros use whomever is currently sponsoring or donating

They are talking about wrist wearables with optical sensors, not EKG chest straps.

Though Polar’s apps are another good example of good data in a horrible app.

Yeah, for heart rate, polar chest strap is most accurate. But for heart rate, running watch is almost exactly the same for me.

Others report huge differences. It could be size of wrist, or who knows what throwing off measurements in some?

> root mean square standard deviation, RMSSD

This is incorrect. RMSSD means root mean square successive differences. Interestingly, if you actually compute what you wrote ie root mean square standard deviation, you will get the same result as sdnn.

That's an interesting assessment

I was curious about this and curious if the measurements made by my smart watch were "good" or "bad", and it does calculate RMSSD (allegedly)

Though it seems while they have some correlation they're all over the place https://www.researchgate.net/figure/Scatterplot-for-SDNN-A-B...

SDNN also might apply to a different time range than RMSSD

As pointed out by someone further down the thread, the value of HRV really only seems to appear in comparing to your own baseline.

Just got the latest Apple Watch, so also glad to hear they perform better on HRV. It seems they get a lot of the little details better than competitors in the wearables space. I read previously about how they likely had the most accurate sleep tracking and also had some special sauce in their GPS handling code that made it outperform Garmin.

Apple had goals to make their these health monitoring features in their watch medically approved, to open up additional markets. For other watch makers, it was a checkbox on a feature list.