Lactate Threshold

Start a run activity, but do not press the Start button.
Press Up to see choices.
Select "Lactate Threshold Guided Test".

Are you sure that your max. heart rate in the watch is correct? You did not just enter a number from an age based formula? Some users have experienced that the watch is not able to detect a lactate threshold if the max. heart rate in the watch is too far from the real max.

Are you using a chest belt heart rate sensor? I don't know about the + model, but earlier Fenix watches could not detect LTHR using the optical HR sensor in the watch.

Ah, found it - thanks.

For anyone else wondering - after pressing up to see choices, you'l see [Intervals] - [My workouts] etc - keep scrolling down and select [Training] then scroll past [My workouts] - [Intervals] - [Segments] to [Lactate Threshold Guided Test].

Did it work?

I failed to mention that my F5X+ has not yet found my LTHR. So far, I can't say that it is an error, though. As most of my runs are at low HR, there is probably only one or two of my runs which would have had a chance of qualifying for an LTHR calculation.

I haven't tried it yet - may try it out on my run tomorrow. I have the Wahoo Tickr X, which I think should work (or does it specifically only work with the Garmin HRM strap?)

When the watch calculates LTHR, it not only measures your heart rate. It also measures your heart rate variability - the small difference in time between each stroke. This is important because the variability will change when you reach your LTHR.

So any device which does averaging of heart rate over time is probably ruled out. Probably also any device which can't either report the time intervals or at least transmit each heart beat with the true intervals. I don't know if any of this could be the case with your Wahoo.

The Wahoo is an optical belt which you wear on your arm, right? So it might have some of the problems that the internal HR sensor in the watch has. But that is only guesswork from me.

The Wahoo is an optical belt which you wear on your arm, right?

No, it's an electrical pulse chest strap HRM.

It worked for me today with my F5X+ and a HRM-RUN. This was the first time I actively tried to get a LTHR reading from the watch.

I did not use the built-in LTHR test. Instead, during the first 3 km of my run I slowly increased my speed so my HR more or less linearly ramped up from 120 to 170 BPM. (I knew from my previous watch that my LTHR should be 162 BPM, so by going to 170 I was pretty certain to have data on both sides of the threshold.)

I think it is important that you ramp your heart rate continuously instead of increasing it in steps. If you suddenly jump from a heart rate far below the threshold to a heart rate far above the threshold, the watch will probably have a harder time detecting the subtle changes in your heart rhythm which are used to detect the threshold.

The instructions in the built-in test are not very clear about this ramping. As I remember it, it will ask you to run for a few minutes in one rather wide heart rate interval, then run for a few minutes in another, wide heart rate interval, etc. So it will be quite natural to increase your heart rate in rather large steps when following this procedure. But I have heard from others that they have had success ramping their heart rate inside each interval.

Example of this: If the watch tells you to run for 3 minutes at 140-150 and then 3 minutes at 150-160, you could choose to run 3 minutes at 145 and then 3 minutes at 155, and that is probably what most people do. But you could also choose to run at 141-143 for the first minute, 144-147 for the next minute, 148-150 the next minute, etc. This would keep you inside the defined intervals, but at a smoother HR increase over the duration of the test. So you will have a better chance of getting data from the HR range just around the threshold.

I did the lactate threshold test this morning. It was about 25 minutes, including a 7-8 minute warmup. I may take it again in a day or two - I think I had audio alerts turned off, and wasn't paying that close attention to the timing when the watch told you to move up in HR interval. As AllenOlesen noted, it starts out at (for example) run four minutes at 125-135, then 136-146, etc. (can't remember the exact intervals). It gave me a slightly lower threshold HR than I had expected, and I was still well below my 'ok this is really really tough' HR level.

I was using the F5+ and the Wahoo Tickr X.

It gave me a slightly lower threshold HR than I had expected, and I was still well below my 'ok this is really really tough' HR level..

If you run for 30 minutes at the highest pace you are able to keep for those 30 minutes, you are supposed to be at LTHR for the last 20 minutes.

So there are several grades of "this is really tough" which are harder than that.

I have the Fenix 5, and as OP I also have never had my watch telling me my LTHR.
I run all sorts of passes during the week: Long runs, intervals, trail, speed and agility to a sum of 50+km a week.
I have used the same watch for over a year now, and never ever have I seen one single LTHR.
(232 activities and 1752KM) Not a single LTHR in that period.

So, I basically gave up on it. I use the HRM strap for all my running, that came with the watch.

I haven't tried the guided run thou, but maybe that is the only way to trigger the watch to measure it, I don't know, as i said, I kinda gave up on it.