I also have a 935, I have not had any issues like that, I have the strap snug so the watch cannot slide around but not overly tight. I read somewhere that tattoos or heavy arm hair could interfere with that ability to get a good HR read.

I haven't had any issues, even while doing intervals (see below). I do strap it on a little tight, making sure it doesn't slide around at all. I find the strap a lot more comfortable than the 910/920 ones though.

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Proud Member of Chris McDonald's 2018 Big Sexy Race Team "That which doesn't kill me, will only make me stronger"
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For the run I rotate it inward. I may be imagining it but I believe I get a better HR reading on the inside of my wrist where the skin is lighter and I have less hair.

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AV8 | Team Wattie Ink Elite 2019

The wrist is a pretty terrible place to read HR. That is just a problem inherent to the design.

If you want all day tracking, it is fine. If you want actual data to train/race to, get a real strap. Chest straps are leaps and bounds better. The scosche optical is nearly as good as those, but some find it way more comfortable.

Where are you wearing it? You need to be about two finger widths above the wrist bone.

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https://markmcdermott.substack.com

I have a 735, not a 935, but as others have mentioned, making sure it's nice and tight does help. For most workouts I still go with a hr chest strap, as it's more accurate.

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While some might have reasonable results from WHR most seem to have problems. I've now worn mine for four runs at varying paces on different terrain and had very mixed results, mostly not good. The most recent was on a road at a very relaxed pace and was probably the worst -https://connect.garmin.com/.../activity/1874883340. The technology is not up to it yet. Perhaps it might be someday but for me it's now back to thickest strap.

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Trust me I’m a doctor!
Well, I have a PhD :-)

Jury's still out on this watch, for me....but....I've had no issues with the optical HR aspects. I use it for all running and all cycling (in broadcast mode). I bought the bundle and haven't used the strap, since I figured out how to send the info. to my 510. I'll likely never use the swim strap.

I raced for the first time with my 935 at IMLP this past weekend. It took an hour and a half into the bike before I got a "real" HR reading. That first 90 minutes it was reading only around 100-110bpm which for the effort I was putting out I know was totally wrong (should have been around 150). Finally about 90 minutes in the HR reading suddenly started showing a believable number.

Frustrating as I was relying on having my HR display as a secondary performance metric to my power meter. Sure I still had power to pace on as always but I like using HR to sanity-check.

You guys just need to have pasty skin and the Optical HR monitor will work without any issues. :) Loose, tight, wet, dry, running, biking, hot, cold. The Optical HR on my 735xt always seems reasonable. I don't wear a HR Strap very often to compare, but I've never had an issue where the HR seemed out of whack with what I was feeling.

It's pretty well established that you should not use a wrist based OHRM while biking on the open road.

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https://markmcdermott.substack.com

Why? I do it, every ride.

My 935 was working FAIRLY well and had been much better than my old Scosche and about as at least as good as the chest strap considering the chest strap's occassional trip into the 200's at the beginning of my runs or on bike descents. Lately however the wrist HRM on the 935 is going into a very stable mode. For example it will go to 157 and then sit there for 20 minutes, which I know is not what my heart rate is doing. Anyone else have this? The inaccuracy is making the entire training portion of the watch useless. I don't WANT to go back to chest strap but I feel like I might need to.

I have had this exact same problem.

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2018 Races: IM Santa Rosa, Vineman Monte Rio, Lake Tahoe 70.3

I've faced different issue, HR reads a lot higher most of the times, even on steady runs. I use the watch very tight, but still issue happens very frequently.

Now always use it with a strap.

Sounds like cadence lock that some people have experienced. Fortunately I haven't with the 935. Got it a few times with my 235 though.

While cadence is much more consistnent than HR, I'd still expect some variation. With these reports, it is always better to look back at the raw data, then to try to remember from checking on the watch throughout the run.

FWIW, when my 235 and 735 fell into cadence lock, they would display 'one foot cadence', thus ~85. Although, I know this is not consistent and some people get 'both feet cadence' or ~170.

Apparently some issues with the WHR have been addressed in the release of software 5.33b for the 935

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Trust me I’m a doctor!
Well, I have a PhD :-)

Following up my previous post here, but I raced for the second time with the 935 and experienced the same "low HR" issue.

At IMLP it took about 90 minutes for it to read normally (it read around 100bpm for the first 90 minutes then jumped to around 150 which is what I was expecting). This time I raced a 70.3 and the heart rate for the whole ride was around 90-100 (expecting around 160), but as soon as I went through T2 and out on the run suddenly it was reading more appropriately.

Clearly this thing has issues with picking up heart rate after a swim?

So I just got my 935, going from a fenix 3 just for the optical heart rate. after reading this I did a couple of tests, I went for a run with both watches on and the entire run the heart rates were within 3 beats of eachother. I am happy with those results. I did not do any rides on bike, however I have done many bike rides with a scothe rhythm and never had an issue with heart rates/