Training effect changes when I download HR data when using only the chest strap HRM with optical HR is turned off.

What do you mean "when I download"? From where to where? Is it possible the activity wasn't synced yet between the watch and the Garmin servers before you downloaded, and the download triggered a sync, so before the download you still saw the training effect from before the activity?

When reviewing the activity you are able to download the HR data from the HRM (download HR). This is what I am referring too. During the activity I turn off the optical HR on the watch and only use the chest strap HRM. When the activity is finished I review the training effect data to see my results. If I "download HR" under the activity the training effect results changes. My question is why? My logic is the result should not change if only the chest strap HRM is connected. So which training effect data is accurate? e.g. -  Before I "download HR" my training effect is Aerobic 3.4 / Anaerobic 2,9 & after I download HR, my training effect data is Aerobic 4.0 / Anaerobic 0.9. This also changes the "Load" data. 

What is the correct method to receive the most accurate HR data?

This is a long time bug with the Fenix6 and the HRM-PRO/Plus that I've complained about to Garmin now for six months but they give zero interest in fixing it.

There are several factors going on.

Yes the download is going to be more accurate than the realtime ANT+ broadcast because data is lost/dropped during the realtime broadcast, this is well known and documented not just with ANT+ but also bluetooth.

They also seem to use a different, better algorithm that looks at HRV R-R heartbeats for analysis after download instead of relying on the HRM-PRO guesstimate during activity.

HOWEVER

The time marker for the start and end of the activity when downloading the HRM data is clearly WRONG.

It is off, by a lot. By several seconds if not more.

My best guess is that the recording on the HRM-PRO starts when you trigger the activity but not start it yet, ie. go into Run mode but waiting/warming up and connecting to GPS, the HRM-PRO starts recording and the Fenix6 uses THAT warmup as a starting point NOT when you push the START button.

Some is really off. Because the time in zones after download DOES NOT ADD UP to the total activity time, it is truncated and starting at the wrong point.

I constantly have  VO2MAX and THRESHOLD runs turned into BASE after download, that's impossible.

Furthermore after they are turned into BASE, there is ANOTHER bug where it does not add the load to the BASE for the week, it leaves it in high aerobic.

This is totally stupid and if Garmin doesn't fix it, the Fenix6 is my last Garmin and I'll have to leave for some other development environment because heartrate and HRV tracking is everything when you are training hard, or old or ill and recovering.

some of my previous threads on this (and I've even emailed Garmin about it)

* https://forums.garmin.com/beta-program/fenix-6-series/bug-reports/i/public-beta-26-xx/firstbeat-effort-load-training-effect-on-26-97b-and-27-00-firmware-for-fenix6-is-clearly-very-broken

* https://forums.garmin.com/beta-program/fenix-6-series/bug-reports/i/public-beta-26-xx/fenix6-29-96b-load-category-still-not-updated-after-hrm-pro-download

* https://forums.garmin.com/outdoor-recreation/outdoor-recreation/f/fenix-6-series/372435/firstbeat-effort-load-training-effect-on-27-00-firmware-for-fenix6-is-clearly-very-broken

* https://forums.garmin.com/sports-fitness/running-multisport/f/accessories-sensors/371079/hrm-pro-plus-has-serious-bug-with-activity-hr-download-time-doesn-t-add-up-zones-are-short

* https://forums.garmin.com/sports-fitness/running-multisport/f/accessories-sensors/364786/is-there-hrm-pro-plus-bug-beta-forum-significant-v8-90-problem-low-heartrate-with-high-hrv-doubles-reported-hr-because-sampling-window-is-too-short

* https://imgur.com/a/htx7SbB

* https://i.imgur.com/lGdQwhy.png

https://i.imgur.com/09UBax4.png

Wow, this is a very through and detailed response, Thank You! I feel your frustration. I only wish Garmin looks into this and fixes the problem. The cost of the Garmin Fenix series is much to expensive not to take this problem more serious. I am an older, injury prone runner who is training for a marathon and use my Garmin Fenix 6 pro as a coach with training peaks. The data is important for training and should work as advertised. It is shameful that the data of the watch isn't more reliable. 

Thank you for your great response...hopefully Garmin will fixes this obvious glitch!   

Well I am a developer in another kind of code environment so at first I tried to report the bug and be helpful and show details, etc.

I've also owned Garmin stuff for a decade now and I've seen lots of things fixed, eventually, though some never.

But then over this past year I realized they didn't care and weren't going to fix it.

Here's another egregious example from today Sunday June 9th, changing supposed VO2MAX effort (very hot/humid) into BASE, which should be IMPOSSIBLE, I mean they are completely opposite ends of the spectrum in effort levels.

The time in zones again doesn't add up after download, it's short

And the BASE is not converted into the weekly total or the 4 week focus summary.

Huge bug, Garmin needs to fix ASAP, not sure if Fenix6 fault or HRM-Pro firmware fault, maybe a little of both, but something is very wrong.

(Garmin both these FIT files from 2024-06-09, before and after, are in my online Connect account and I give full permission to examine them or any others I've been uploading for the past months to help fix this)

https://i.imgur.com/ijp2Y4g.png