VO2max drops after VO2max workout

7017726 said:

a workout that increases VO2max means VO2max actually increases.

In long term it should, but expecting that a VO₂max workout will increase your VO₂max instantly is a bit overoptimistic. Wait a month or two (or even longer) continuing your training, and see what effect it had.

I agree. I just didn't expect it would drop, especially since the watch indicated the workout was improving my VO2max.

'Real' VO2max is a function of cellular and respiratory development and cannot respond instantly. 'Estimated' VO2max provided by calculation can change in response to the factors that make up the estimation. Simply put, 'Real' VO2 max takes time to really adjust; it is a chronic measure of fitness. Unlike estimated VO2max which is erroneously used as an acute indicator of fitness.

7017726 said:

especially since the watch indicated the workout was improving my VO2max.

The workout benefits are estimated based on rules and thresholds of various metrics and their dynamics during the workout. The kind of workout you executed is expected to have long term benefits on VO2 Max. Generally, performing threshold and higher intensity workouts is expected to increase VO2 Max, complementing long easy ride/runs.

In parallel and independently, the watch estimates your VO2 Max after every qualifying workout, and this estimate will reflect the short term impact of conditions, recent training history, etc.

If you continue training with balance and proper recovery, there is a good chance your VO2 Max will increase. If you are a well trained athlete, you may plateau your VO2 Max, or you may just bet/get too old to sustain the rigor of the volume and intensity required to increase it further. If you over train, the accumulated fatigue will lead the watch to lower your VO2 Max estimate.

If I read you right, you're saying workout benefits and current VO2max are calculated along different metrics, which makes sense.

I don't think I was overtraining, though the ride put me at the top end (still Optimal) of my current Training Load.

But age may be a factor. I'm 69 and may be plateauing. While my VO2max increased regularly from 49 to 52/53 from February thru June, it's been slightly up and down since then despite continuing to train. I kinda expected this to happen (reach my limit re volume and intensity).

Good insights.

VO2max changes based on the ratio between your HR and pace. 

If your HR was a bit higher, maybe due to fatigue or whatever else can affect your HR, the algorithm is considering like you performed worse, aka VO2max going down.

Getting a VO2max label doesn't mean your VO2max is improving...you just did something that had the effort or the correct setup to be considered a VO2max workout.

Just to avoid confusion, are you talking about something similar?

If the answer is positive then it's not telling you you that your VO2max is increasing...it's simply saying that your effort should increase your VO2max, in future, in 2 months, in 2 days, who knows when but it's not directly tied to VO2max changes.

Much as I was thinking. In the event, both my pace and HR (and power, both average and normalized) were higher than earlier rides.

This.

And during my last threshold workout my HR stayed at 120 (I love how Garmin measures HR), instead of ~160, I guess, for a prolonged period, so my VO2max increased, yay!

I am happy with my new VO2max now :) - it is all fake now, but who cares ;)