I struggle in the same way. I just gave up on caring about anaerobic effect. The only time I ever saw anything in even the 2 range was when doing half mile hill repeats (and this wasn't even a regular occurrence). I would have to stop after the downhill and let my heart rate basically return to near rest before the next repeat. So anyway now I don't worry much about that number. I just do my workouts. It's getting me to where I want. Sorry I can't provide more help. Good luck in your training.

And that's the crux of it. The Anaerobic Effect is determined from a combination of work effort and recovery, not just simply from a hard effort. Remember too, anaerobic means without oxygen so for best effect your intervals should be less than a couple of minutes in duration at close to max effort with recovery between. The amount of recovery and the degree to which your heart rate returns to close to resting levels will impact on the anaerobic effect returned.

for me too, most of my training i can't reach anaerobic for Garmin Ecosystem, but i am sure, i am, same situation on Edge 830, not only on FR945!

Is there something wrong in the Garmin Ecosystem, cause Trainingstatus is sometimes totally weird now too?

Posted directly from Garmin Connect:

Anaerobic training effect uses heart rate and speed (or power) to determine how a workout affects your ability to perform at a very high intensity. Repeated high-intensity intervals of 10 to 120 seconds have an especially beneficial impact of your anaerobic capability and result in an anaerobic training effect.

With that being said, I personally never found anaerobic all that reliable seeing the same results in nearly every activity I have ever done, regardless to effort, duration, level of fatigue, HR zone, etc. The watch almost always returns a 2.0, or right around it.

Week one doing a Lifting routine interspersed with HIIT and intervals or a run/bike ride and every single workout is 2.0. Adding weight or pushing my HR into that anaerobic level for extended periods like the app specifies and I end with a 2.0. End of a two-month cycle of hard work; each workout is a 2.0 even to the very last workout. They are no different, start or finish, no matter the kind of work I do, it is always the same.

I can even cheese the anaerobic number a bit if for some reason it was low (like around a 1.0), by doing a set or two of bicep curls. I'll jump a whole entire point in 30 seconds of curls whereas nearly forty minutes of lifting (in this case a shoulder routine with a 30 HR spike at the end of each set) won't budge. That last one is rare on the 945 but happened all the time with my 935, but I CAN do this with the 945. 

For the record: I always use an HR strap when working out/running/cycling, and always a Garmin strap. My HR zones are perfect. One last thing, this is not a complaint from my end, just speaking matter-of-factly.

I've also given up on that metric. When doing 200s or 300s it gives me very small numbers. I get numbers way higher (well way higher means up to 2 :-) :-) ) when doing my weekly *long run*. Which is a steady effort <75%HR and most of the time (in sense of most of the runs not within one run) I do not even touch LTHR.

But also not complaining, it's like it is. I wouldn't use it to control my running anyway.

THANK YOU everyone for the thoughtful replies.  Its good to know I am not the only one.  Ill just ignore the Anaerobic Effect.  Oddly, I find the Aerobic Low and High readings to be pretty accurate and actually useful. 

Dr Phil said:

The amount of recovery and the degree to which your heart rate returns to close to resting levels will impact on the anaerobic effect returned.

So i think that is perhaps my problem.  I typically add some hard hill climbs during my easy runs where my heart rate is in Zone 2 for me.  So it seems the key is to let my heart rate decrease more between the high intensity sets. I will try that this weekend at the track and see what happens.

You can look at a similar post in the Marq forum with some great advices of HermannB of Firstbeat.

In short, you can try also these things:

1- check your max heart rate in the watch and in Garmin Connect. Are you sure it's correct? To have a good training effect I changed my max heart rate from 192 (max heart rate that I had two years ago) to 182 (max heart rate that I'm having now in some training session). Some users reported that their watch changed their settings and heart rate was too high.

2- check your heart rate lactate threshold LTHR. I used the value of june 175, but a recent test showed that now after the summer the value is 162. I lowered the value.  In the interval session today most part of the run was in the red zone Z5, while in the other interval session I did was in Z4 orange.

3- three sprints of two minutes could be not enough.  The anaerobic effect is greater when starting from a state where the heart rate is low, so the aerobic component is not contributing much.

4- it should be not necessary, but today and last week I got a anaerobic training effect of 4 (today with a 10x1km session  with 3 minutes rest between intervals) using the chest strap. I will check in the next weeks if similar workouts produce similar anaerobic training effect also with the optical heart rate.

Take a look also at  this other thread in the 935 forum with other good posts of Hermann B 

Anaerobic training effect broken on 6.00?

Finally ,I have to add that in order to get high anaerobic training effect I have to do hard workouts (for example 15x400 m or 10x1000).

If I do  three or for sprints at the end of a normal steady run, I get anaerobic training effect = 0 or under 1

Just a quick follow up...today I ran my usual 6.5 mile route at my normal Zone 2 heart rate, except for 2 short sprints I always do at the end of the run where I hit Zone 3/4.  Usually, Garmin either doesnt register an Anaerobic effect or very little < 0.0.  Today I actually got an Anaerobic Effect of 1.6!  I wonder if its related to the fact that I havent run in 4 days?  Usually I only take 2 days off between runs.  

Hi, I swim around 2.5km or 3km per session. I usually get an anaerobic effect around 2 or 2.5, and a low aerobic effect above 3. However, my problem swimming is that I don't get any high aerobic effect. I guess it depends  on the kind of sport sessions we're doing.