Lactate Threshold test has failed twice - what structure in my run do I need to do to get it to auto-detect?

Standard issue nothing new.

After long time i give up

https://forums.garmin.com/outdoor-recreation/outdoor-recreation/f/fenix-6-series/208292/lactate-threshold-test-on-the-garmin-fenix-6x

For me, doing interval training (400m) in Zone 4 with a HR band does it for me. I have been doing intervals with the OHR on the wrist as I don't like a band, but adding a band on my Zone 4 intervals has actually given me new (and improved) numbers. I last did a real Garmin test 6 months ago.

I've had bad luck with the structured LTHR test too, but recently I figured out how to get it to work (for me.)

First if you have a maximum HR and/or LT configured, make extra sure that it is reasonable or even manually set it a bit low for what you think you can do, I accidentally ran once with my max HR set too high and it tried to kill me.

Make sure you are well rested, take a day off, do an EASY run the next day, then take another day off, then on the 4th day, do the structured test.  If you're still feeling frisky after the test, then you can do extra running and wear yourself out.

For the first 2 or 3 intervals, run at the LOW end of your HR.  For the warm up, maybe just walk or light jog, when you hit the lap button to exit warm up, you should be breathing easy.

For the next couple of intervals, try to run at the low to mid end of the listed HR range and DO NOT PRESS THE LAP BUTTON until you are totally done.  Once you start to struggle it should notice and give you shorter intervals of 3 minutes instead of 4.

I am of course just 1 person, so I don't know if this will work for you, but it is the only way I've been able to get the watch to detect my LTHR.

Thank you!! Just as a set by itself? About how many are you doing? 

Hahahahaha I upgraded from Fenix 5 to 6 and for whatever reason it defaulted to a 202 max HR. I did not know this. I tried to do a LTHR and thats exactly what I told my husband - “this watch just tried to kill me!!” 

so unless I’m doing something wrong, I don’t have to push the lap button to proceed through the different intervals. For example, with a maxHR set at 177, it gave me 4 mins at 139-148, 4 mins 149-158, 4 mins 159-167, 3 mins 168-177 and automatically moved through. I stayed in the correct zone throughout each interval. The only time I pushed the lap button was to move from warmup to the first interval. Am I doing it wrong?? Thanks so much for your help!

I've had the same feeling a few months ago, thought the watch was trying to kill me and the test failed. 5x plus seemed to work, 6x did not

Warmup zone 2 for 10 mins and then 10 400m intervals in Zone 4 with 2 mins rest after each

Perfect, thank you!

It's possible that your maximum heart rate value is set too high. It drops over time as you age and is influenced by temperature.

There are guided workouts on the internet to discover your current max heart rate, though in general it's a pretty unpleasant process no matter how you do it. 

Ultimately you'll do the test and the result will be exactly what you're expecting and what you probably already know - the heart rate at which you know instinctively you can do for long periods and above which you fatigue quickly.