Alway anaerobic shortage

Do rather some interval training. You cannot really achieve anaerobic effect when you stay ~20 minutes in Zone 5 - that's not anaerobic mode. An athlete can sustain an anaerobic mode for around a minute. You rather need short bursts of maximal (or near-maximal) effort 

A 5k is typically a VO2 Max kind of effort

https://www.firstbeatanalytics.com/en/features/workout-labels/

Let's say you ran your 5k at about 110% of your threshold speed, your anaerobic intervals would be between 150% and 200% of your threshold speed, around 60s with 5mn recovery. Repeat 6 to 10 times.

Here is a biking example Anaerobic - Aerobic 3.7 - Anaerobic 3.2

Intervals were 90s, 6mn rest at 150% of FTP

A 5k is typically a VO2 Max kind of effort

https://www.firstbeatanalytics.com/en/features/workout-labels/

Let's say you ran your 5k at about 110% of your threshold speed, your anaerobic intervals would be between 150% and 200% of your threshold speed, around 60s with 5mn recovery. Repeat 6 to 10 times.

Here is a biking example Anaerobic - Aerobic 3.7 - Anaerobic 3.2

Intervals were 90s, 6mn rest at 150% of FTP

Totally agree with trux and Etupes25

If you can sustain an effort for 20 minutes, then it is primarily aerobic in nature.

It is really only VERY HARD efforts that involve your muscles working anaerobically, hard enough that you can't really sustain it for more than 20-30 seconds. Sprint intervals are an ideal way of creating anaerobic benefits.

It's hard to get anaerobic running benefit that Garmin watches acknowledge. If you do sprints, you will need a chest strap to react to the quickly rising HR and even if you have that, the GPS/INS can't react to fast pace changes. You'll just end up with aerobic benefit in the watch, even though in real life you did get anaerobic benefit from the exercise.

Have a look at your max heart rate, if it is realistic. I had auto max heart rate detection on for a while, and this raised the bar for 'anaerobic benefit'  so high that I could not ever get any.

My max HR is auto detected, but realistic. I have no trouble getting my HR up to the required values during the sprints. This is if I use a chest strap. Not a chance with watch wrist HR, its accuracy and reaction time is poor.

My contribution is that I would not dare to say that the calculated AnTE is too low, but I say something smells.

Today I decided to check it after each of my sprints of 30-40 seconds.

After my first one it was 0.1, after the second still 0.1, after the third still 0.1. I was on the verge of dying, I had been trying so hard.

While jogging after the third all-out sprint I decided that having gotten back home I would be writing a sort of bug report saying that sw 24.00 must have ruined AnTE calculation, but after my 4th sprint which was again 30-40 secs long (its pace was a tad weaker)  AnTE jumped  immediately to 1.8. I had some more sprints, but it was impossible to go over 2.5 

So again I don't dare to say AnTE is fake or sort of, but with my Fenixes I have seen too many pecularities regarding it, while at the same time I much much less regarding AeTE.

Some of the pecularities seemed to be logical.

CASE 1:

when I have all-out  surges cycling with powermeter I accept that the increment of AnTE is rather low, because thinking in terms of "exercise physiology" of cycling I am at the top of in my neuromuscular zone  and not in the anaerobic zone called Anaerobic Capacity. I mean based on pure wattage. In other words I can assume that my Fenix 6X (or rather the Firstbeat algorithm) is clever enough to use the power data in the estimation of both AeTE and AnTE. Certainly I am not sure about it, but at least I was able to deduct some logical explanation.

Other pecularities are not logical, and frankly speaking they are unacceptable for me. 

CASE 2:

while AeTE is increasing by 0.1 from 0.0 to 5.0 if you run long enough,  AnTE likes to leap up sometimes by 1.0-2.0, see my example above. So in the background one's AnTE may be accumulating, but it is voted and confirmed typically in "batches".

CASE 3:

another pecularity is that there are AnTE numbers which behave like strong and thick walls, so you have to do more extra to get over these specific numbers,

CASE 4:

finally for me the biggest pecularity of AnTE is its scaling from 0.0 to 5.0. I mean whatever I do, either I increase the number of sprints, or the duration of those sprints, or do shorter, but really all-out sprints, it is almost impossible to go over 3.5. And as I remember I was never able to get >=4,0. I mean when running. As regards cycling I got some 4.x, but x was 0, 1 or maybe 2.

Personal data:

I have relatively low aerobic capacity, my IVC/IVC pred. was always measured between 66-72%, FEV1/FEV1 pred.  67-73%. VFCex/FVCex pred.was 61-70% etc. My Vo2Max was always below 50 in my whole life, typically between 45-47, but I can complete a training session to get 5.0 for AeTE. 

On the other hand I have some sort of relatively good anaerobic "skills", my lactate threshold is 169-170bpm being 90-91% of HRmax. I have relatively good lactate tolerance measured in lab directly from blood, but I cannot complete a training session to hoghly impact it.

So you can say that these are exactly the reasons behind my AeTE and AnTE numbers.  BUT my Fenixes never gave me high AnTE, not in those early periods before these watches figured out my LT. Full stop.

mcalista said:

It is really only VERY HARD efforts that involve your muscles working anaerobically, hard enough that you can't really sustain it for more than 20-30 seconds. Sprint intervals are an ideal way of creating anaerobic benefits.

I wanted to react to this separately, because my observation is that if I really want my actual AnTE number  to elevate I have to sustain the sprint for 30 seconds. So I guess that there are not just "strong and thick walls" as regards specific AnTE numbers, but also some sort of cut-off numbers as rgards the duration of one sprint-block.