VO2 max is a measure of oxygen uptake and distribution. Your VO2 max will transfer over. However, if you're measuring VO2 max in some ancillary way (not a stress test machine) then it will appear differently since you're level of effort will vary based on your technique dor the chosen exercises.

That's why there are separate Cycling and Running VO2 indicators

You won't be a 55 Vo2 runner if you're a 55 Vo2 Cyclist, but cardiovascular fitness does cross between those activities to an extent

Your indicators will be different, but your actual O2 uptake should be essentially similar. Any difference should be due to technique (if you've optimized your breathing/rhythm/etc for one but not the other).

This is incorrect. VO2max is influenced mostly by muscle recruitment - different muscle group trained would be a hindrance if anything

"VO2max is influenced mostly by muscle recruitment"

This is incorrect - the primary factor is heart and lung function. Yes, muscle training can play a factor, but is relatively minor compared to that.

Lung capacity is genetic and heart is only trainable up to a small limit bc there’s a layer of fascia around it preventing further expansion. That’s the “supply” side of vo2. The “demand” side is the recruitment and efficiencies you’ve developed which are exercise dependent. This is why you dont see a lot of cross training (except for recovery) and a lot of z4 work in the professional training programs

My VO2 max from running definitely transfers to cycling. Doesn't mean I'm an amazing cyclist but at least cardio isn't my limiting factor.

It transfers to an extent. Lance Armstrong marathon PR is 3 hours from what I read, which is a great result for an amateur runner but totally unimpressive for a professional endurance athlete, let alone one of the best in history. I was actually a bit surprised that's the fastest Lance could do and to me it shows it doesn't cross over that well.

Not surprising at all. Vo2 in general is not great metric for elites and specificity is king. Which is why you rarely ever see elites cross train much

I wonder if running crosses better to cycling than vice versa, I think it might cross a bit better because running involves more (pretty much all) muscle groups. On the other hand there might be a lot of technique in cycling that takes a while to learn so idk.

Is that true? Back when I was a crew gremlin, my VO2 max as measured via erging was off the charts. But I could apply that cardio capacity to things like running/biking/swimming pretty effectively.

There's famous studies about people testing VO2 max with one leg trained exercises,then looking at VO2 max with non-trained leg.

https://onlinelibrary.wiley.com/doi/10.1111/j.1748-1716.1976...

It's true in the sense that you'll be better optimized towards the activities you train in most intensely in regularly, but you do develop transferable gains as well. It's just not 100% transferable.

I think the GP's comment is just easy to misread.

Running down the street at top speed, i.e. for a minute or so, is an anaerobic activity.

Even sprinting 100m takes 60% ! energy from aerobic system.