Hi there,
I’m using TR now for a couple of weeks (FTP program). I (57/M) do all the workouts as planned bij TR. Love the integration with Zwift!
The issue I’m having is that my VO2max went from 44 to 36! I know there is no VO2max training in the build phase but I’m curious if that’s normal?
How are you estimating VO2max?
I’m assuming that it’s the Garmin?
If so then I’d be looking at the integrity of the OPs heart rate and power data.
How are you measuring vo2 max?
What plan are you on that has no vo2 max work during build phase?
If you are using something like garmin to estimate vo2 max then it will drop if you are not feeding it maximal efforts.
Sorry, I use Garmin which gives me my Vo2max. I’m now at the Masters General Base I which consists only some endurance and sweet spot workouts, 5 days a week.
To be clear; I feel great but I’m a little worked that my VO2max drops that quick.
Garmin hasn’t measured your VO2max it has guessed based on the data you’ve given it, you’re now giving it different data from endurance workouts. It probably hasn’t dropped drastically you just haven’t given Garmin the data to support guessing as high a value.
It sounds like you are still in base phase? Not build?
If so then having no maximal efforts to feed the garmin algorithm makes sense. It drops your vo2 max because it has no maximal data. I would expect that to change once you go into build phase.
That feature is only worth looking at with some mild curiosity and for entertainment value.
Yes, I’m in the Masters General Base I phase. After this there’s another base II phase. I will wait but I’ve riding with Zwift for the last 2 years and it only went up.
I know… but it’s concerning when it suddenly drops, without an obvious reason, isn’t it.
No, it doesn’t. 
No it’s not concerning because the Garmin measurement is flawed.
Endurance / base training is VO2max training. In fact all training increases VO2max in untrained subjects.
Short of going into a lab and testing your VO2max there would be no way to know.
I suggest you research how the Garmin calculates VO2max. That will tell you what inputs you need to show it in order to get the best guesstimate from it.
Searching I come up with:
Garmin calculates your VO2 max estimate using an algorithm that analyzes performance data like pace and heart rate during outdoor, GPS-tracked activities, along with your personal details such as age, gender, weight, and height. To get an updated estimate, you need to complete an outdoor run or walk for at least 10–15 minutes at a consistent, moderate-to-intense pace that elevates your heart rate to at least 70% of your maximum.
So the first thing you need is a good quality chest HR monitor and you need to know your max HR and then you need to do some efforts.
Garmin uses the First Beat algorithm:
It’s not concerning because it can’t be real.
I’m in the camp that finds Garmin VO2max useful to trend… and although there are definitely detractors the consensus is that the number generated is fairly accurate.
However, in your case, I think something has changed that is skewing the number.
Garmin VO2max uses your weight, your heart rate, your heart rate zones and your power numbers.
If you have changed any of those or your method of recording them then I’d be checking that the numbers you are giving Garmin is correct.
Of course it’s just as likely that it was wrong before and is now correct.
TL;DR - something has changed but it’s unlikely to be your fitness
But it still hurts!
Same. Mines trending down, not surprised I’m lifting 3 days a week and barely doing any TR workouts.
The variability in the Garmin VO2 figures are nonsense, especially if you use different power meters on different bikes, or the weather changes from hot to cold. Mine drops to 50 if I ride the mtb, goes up to 52 if gravel ride or do a lot of z2 rides, drops in the summer when its hot and goes up in the winter when it’s cold.
It seems to have the best part of naff-all too do with my actual fitness tho. It never changes out of that 50 to 52 region no matter how much, or little, training I do! 
So the first thing you need is NOT an optical HR monitor.
Quality chest strap is required.
Yes, I was thinking the reverse of optical like a good chest strap but wrote optical. One probably wants to avoid watch HR but even those are getting better.
Basically you need to do anaerobic work to maintain your Garmin VO2max… if you don’t as in many base plans it will drop.
I do have a Garmin chest strap.
Your VO2max isn’t dropping at all over the course of a couple of weeks if you’re training at all. Your Garmin Estimated VO2max is dropping, and that’s not really anything that should concern you at all because it’s not a precise nor particularly accurate estimate in the first place.