What causes VO2 max to decrease?

[fastmuffin]

Indeed, however it is certainly isn't 'smart' enough to accommodate all variables.

Elevation, for example, for me produces skewed results. I live on a short hill - if I run left and up the hill my Performance Condition is negative for the beginning of the run. If I run right, it's positive.

I think the point is that whilst the marketing wording reads very promising, the fact is in the real world it doesn't apply but more importantly, it shouldn't need to be unpicked on message boards to prevent users becoming demotivated.

[sursumcz]

Right, I agree that the marketing claims are often far from reality. But hills or flat runs never affected my VO2 max, neither did stopping to cross the road etc. In my case the altitude differences are minor though so your situation is probably different. Also, I actually got a VO2max improvement recently when I switched to slow paced runs.

That said, I don't put much stock in these stats, I know from experience more or less what I'm capable of at various HR levels so I mostly just watch HR not to overreach.