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macrumors member
Original poster
May 6, 2019
Hi all. I noticed that sometimes my Apple Watch 8 (and even my old 6) doesn’t always register my heart rate during workouts. It’s grayed out. And the band is plenty tight to be able to register and no tattoos. I’ve noticed it more when I’m not warmed up, like during chilly bike rides or using the ‘Other’ workout when I don’t get my heart rate up as much as I would for a vigorous workout. I’ve also tried cleaning off the rear sensor, but to no avail.

Also, if I go on a 40 ish minute bike ride and it measures my heart rate, say 10 of those minutes, how can it accurately compute active calories burned? Estimating?

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macrumors 6502
Sep 25, 2022
Too tight doesn't necessarily help either. I have noticed that certain bands seem to help with this. Nylon and sport type loops without magnets seem to work better for me, but its totally anecdotal.

I don't think during workouts its necessarily a constant monitor, I'm guessing its every 10 or 20 seconds or something, I don't know.

You for sure can't get too sweaty, and having the right position on your arm is important. I've havent had a ton of issues with this. You're not a vampire are you?


macrumors member
Jun 26, 2014
I always have issues with wrist-based HRM...every watch I've tried. I'm a caucasian male with normal pigmentation and no tattoos. I'm fairly fit, with little excess fat (especially on my wrist).

My previous Apple watch, series 5, could NOT get any good HR measurements no matter what I tried. Often in a run or some other very normal activity (hike, walk), the HR data would be greyed out.

I now have the Series 8, and I have moved it to my other wrist. Now- almost always I get consistent, good lock and reliable HR data. So much, in fact, that I stopped wearing my Garmin FR 255, which was giving me very inconsistent HR measurement values.

I don't know if it's the AW Series 8, the move to the other wrist, or OS9...but something is better and I'm happy about that.

But I do know that no matter what I tried before, nothing for me seemed to work.


macrumors member
Oct 20, 2018
I am experiencing similar greyed-out heart rate measurements on my AW Ultra. I have tried alpine loop, sport loop, and sport bands so far during my recent outdoor runs. All ended with missing heart rate data. I get heart rate monitoring for the first half of the run, then it stops.

I always have continuous heart rate measurements during indoor exercises (indoor cycle, weight training).

I don’t know what exactly is causing the problem on outdoor runs, but it’s annoying me a lot. Any suggestions?


macrumors regular
Jul 2, 2007
I’m also getting inconsistent reads from my ultra. More after than my series 3 did, my ultra grays out the Hr for several minutes, multiple times during runs. anyone else see this or have other suggestions. Been trying various adjustments but nothing helps. How often varies by run so not consistent.


macrumors 603
Mar 10, 2016
I think it’s harder for it to get a track on your heart rate while your heart rate is rapidly changing pace.

I usually start the heart monitor just prior and let it get a view of my heart rate, then begin my exercise.

Anecdotally that seems more reliable. But I also went from the SE with 2nd gen sensor to the S8 with 3rd gen.
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macrumors 6502a
Apr 13, 2012

I did a quick ab workout to put it through its paces. Th is the heart rate graph. There’s a lot of empty space in there. I am not sure what that’s all about. I am currently running two apple watches, one 41” and one 45” because I can’t decide which size I like. This was the 41” and tomorrow I will do the same with the 45” to see how it goes.

I am leaning toward the 41” right now because the GPS seems more accurate.


macrumors 603
Mar 10, 2016
This was a shorter and more chaotic workout. Some body strength stuff and some weights. I’m not hugely into that to be honest so not a great frame of reference but for what it’s worth mine appeared choppier too.

The earlier choppier section was push ups IIRC.



macrumors member
Oct 20, 2018
I believe I have found the cause. Since Ultra is heavier, it slightly shakes on the wrist during my outdoor runs, which impacts heart rate more and more when I start sweating.

I tried another outdoor run with a super tight fit using my old sport loop band. I have finally got almost perfectly consistent HR measurements. My wrist size is 155mm while sport loop supports minimum 145mm wrist size. However, with my super-tight fit, it is now too long for my wrist!

I placed an order for small/medium trail loop, which promises compatibility with 130mm or more wrist sizes. I think, I’ll stick to the trail loop during my outdoor runs once it arrives.


macrumors 68030
Aug 6, 2022
I also will lose heart rate while hiking. I readjust my watch and it comes back, but from the looks of the graph, it drops and reconnects without my knowing it. It only happens for a minute of so but no matter how lose of tight my HR monitor drops out. This happened just as much on my SBTi series 8 so it is not just the Ultra with the problem.
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