VO2Max Fluctuations due to Low Intensity Runs

Switch to Trail Run activity when you don't want your runs to affect your VO2max.

It is interesting, because low intensity training shouldn't trigger VO2max calculation at all.

same for me, each time I do an easy run, I become unproductive and my VO2 max drops 1 or 2 points ... 

and then of course when I do a workout, my vo2 max goes up again. 

I may try the trail activity way, but it's not really a proper solution

You will get a performance condition and VO2Max calculation after a few minutes in zone 2. Keeping it in zone 1 will avoid the calculation.

First let's be clear. It is a VO2max estimate. That is important, meaning IRL if you run a fast run one day and slow the next, your real VO2max did not change. 

So, what is it looking at, how hard your heart is working at a given pace, throw it in a table/equation. So what HR would we expect a person of VO2max of 52 running at a 6:30 pace. What HR for a 9:00 pace. 

A. You may simply have a HR that tends to be a bit lower than expected/average/typical at the hotter paces shorter distances, and a HR that runs a bit higher at the slower paces longer distances.

B. More important/relevant perhaps, there are things that can bump your HR at slower paces. Your form, efficiency, cadence, and breathing can all increase your HR since they are perhaps more or less ideal at different paces. I'll give you a simple example for me, breathing. Since it is so easy to run at these slower paces, sometimes breathing takes a back seat without noticing and I see my HR creep up. I pick up breathing, and it goes down. If I didn't pay attention, I wouldn't notice, because the pace is so easy I'm not tiring perceiving myself working hard, but there my HR would sit working harder as far as the watch knows, giving me a lower VO2max when the watch sees I'm working harder for that pace. It could just as easily be some other running form thing that is having you work a little harder or less efficient. Perhaps running dynamics, such as vertical ratio, could help you identify some of these.

C. On longer slow base runs, assuming a constant pace, make sure your HR is mostly plateauing after warming up and not on a constant significant creep up, or shooting way up at the end of runs. Otherwise you may be going to far/fast and reaching your endurance limit and at that point be recording rather high HR for a relatively slow pace.

I do think you know/realize a lot of this, but you need to accept that it can lead to inaccuracies. It is very important when observing these things the watch really only has a limited data set, pace, and what your heart is up to. Let's be real in the first place form, breathing, caffeine, sleep, stress, temperature, wind, road conditions, sleep, hydration, surface, food, etc. can (and does) impact that HR/pace relationship messing up your regular VO2max reading (the one you were happy with).

If you are seeing discrepancies then I would say either:

A. Compare slow runs to slow runs over time.

B. Just compare your trend within the time period and mix of runs you are doing.

I know it is annoying to see your VO2max and stats drop, but all that is really relevent is the current trend at the paces you are running, even if inaccurate toward the faster predicted paces. Meaning as you watch the trend it it is showing you at those base building lower HR paces is your heart working (hopefully) less hard over time. 

Why this is ultimately irrelevant, even if you find no solution. As soon as you go back to normal training, your numbers will snap right back and you can move on with your nice healthy base. You got the same benefit of those 2 hr runs regardless of Garmin saying they were a lower VO2max and annoying you.

All that to say, I wouldn't disable it since you can still see a trend of improvement. If you must, then use trail run. I'd suggest you duplicate it and set it up as Run 2 or something if you actually use trail run so you can set both up how you like.

That's a really good explanation but I know how it works (it uses HRV also) and I think it works badly !

I already posted about it here : https://forums.garmin.com/sports-fitness/running-multisport/f/forerunner-945/193945/light-runs-always-unproductive-while-intervals-always-productive

Honestly, I don't care much about the VO2max estimate, the real problem I have with that is the training status linked to it.

Each time the watch says unproductiove, it stays like that for a while (it needs a really good workout to get the VO2 max up again and become productive again).

that means most of the time my status is ... unproductive. just because I did a slow run one day.
It's really annoying because I know it's not true 

I finished a 20 mile run today at a decent pace and my training status went from productive to unproductive after the run lol. Vo2 max also dropped a point. Hr, pace, distance etc seem like they were all recorded properly, obviously the 20 mile run was a little slower than say a 6 mile run but still why the unproductive status and drop in VO2 max?! 

After my easy run last friday (zone 2 for 45 min),my VO2-score went up 1 point.

I was about to write 'don't pay too much attention to individual datapoints, it's the trend.. etc' but others have already done that. And then I saw that my VO2Max was incremented by 1 point after a two hour 'zone 2 run' that was supposed to be real easy, but turned into a horribly slow struggle with very high winds. Surprising - and it feels good to be "rewarded" for the high perceived effort by this one datapoint.