Abnormal heart rate when doing "Cardio" workout Fenix 8

If you care about HR accuracy, you'd better wear a chest strap. It is pretty common for the wrist HR to show false spikes of HR during workouts. The new sensor on the Fenix 8 is supposed to alleviate this further, but if you care about accuracy, ....

When I forget my chest strap at the gym, I get almost systematically several issues with HR data. The watch doesn’t catch the hr increase towards the end a set, and/or reports huge spikes of a sec or two during the sets. Calories might end up in the ballpark on balance, and anaerobic effect also, but it is serendipitous.

Note the Fenix3 doesn’t have alerts for abnormal heart rate, but the Fenix 6 does.

Any workout with a lot of wrist movements or flexion makes it super hard for an optical hr on your wrist to get a good reading. So if you do CrossFit, HIIT or strength training, you might want to invest in a chest strap or an armband hr.

If you're going to use only your watch, make sure it's a little higher on your wrist and very tight, that should help. Try to keep your arm straight by your side for a few seconds in between each set so the watch can catch up. 

It's not a Garmin problem, any watch from Apple, Suunto, Coros or Samsung will have a tough time tracking heart rate during those workouts.

I have the same issue on f8 47 amoled after switching from F6Pro

https://forums.garmin.com/beta-program/fenix-8-series/fenix-8-family-enduro-3-fenix-e---public-beta-reports/i/public-beta-11-8x/11-87b-f8-47-amoled-hr-value-is-higher-by-20-points

*copy from report

Hi. I've had a Garmin fenix 6 sapphire for three years. All these three years I did mostly strength training. And during strength training my heart rate was up to 150. But never higher. The working range is 120-140.

Now I have a Garmin Fenix 8 47mm AMOLED and since using them my heart rate has gone up to 140-160 and peak 170+.

It seems to me there is some kind of problem

Hi! Today I checked the HR value on the F8 and the HR values on the external device.

Sometimes there are spikes, when conditionally heart rate is 130, it shows 120. Or real value 120, F8 shows 145. But if you give it 10-15 seconds, the HR values become normal.

Moreover, relative to F6 (according to my memories), it shows more accurately at a heart rate of 150+. But F6 have never been wrong for 20 heart rate values.

Conclusion: there is a problem, definitely messes up the “maximum HR” and for sure in other indicators. But on average it measures normally

And yes, the strap I have is the nylon velcro strap from the F6, the F6 is 47mm, same size as the F8. That's my point, I wear the F8 exactly the same way, with exactly the same fit against the skin

Mine has been ready extremely high vs a chest strap.  Morning walks on the treadmill show readings in the upper 120s or low 130s when typically they should be +/- 100.  This is throwing off my effort and recovery times significantly.  Older Garmin watches match much closer than the F8.  I've seen spikes going ridiculously high as in the 180's which at my age I haven't seen in years.

Don’t ignore it! Before you dismiss this as a watch fault, just make sure. It may be telling you the truth. 
Try a chest strap or go get tested. 
“He complained recently that his watch was over reporting his heart rate” (Heard by me at the funeral of a work friend)

Also worth knowing: there is an ap which allows the watch to record wrist recording and strap at the same time. You can see both live on watch plus it records both against the activity. I ran it with my F6 pro. Was interesting. Matches pretty well. But 8 second gap from strap to wrist. Strap was live with wrist following. So you start a climb, you first see pulse rise with strap then watch match 8 sec later. Same when you slow down. 
As I say. Interesting. 

I hear you but in this case I know it’s the watch.  I run with a strap.  After my workout I look at my hr and it’s higher than when I was running (strap disconnects from watch after ending a workout).  I reconnect the strap to the watch and the hr immediately goes way down.  It’s weird but my resting hr is still close to previous Garmin watches but once my actual hr goes up beyond that it’s like the F8 has some multiplier which sends the hr way above where it should be.  

Ive had a apple watch 2 ultra and when i'm at F45 working out i also have a strap i wear which links to the F45 monitors, that heart rate strap is very accurate and is almost always the same as what my apple watch 2 displays... a few weeks ago i bought the fenix 8 51 amoled and the heart rate is low when i'm working out hard (cardio) and seems to be high when things slow down, its been doing this for a couple weeks now and today i wore my apple watch 2 and got near perfect (normal readings)... absolutely something is wrong with the heart sensor on this new watch... running the latest software...