Cross-Posted from Physical Exercise, as I suspect this Q is a bit more bio-science focussed than is appropriate for that StackExchange
I play hockey (field hockey) at a slightly-below-elite level. I train (but don't play) with players who play in the UK National Prem.
I, and other players I play with, and coaches I've trained under, have always talked about us and our brains being "oxygen-starved", when you're playing beyond your fitness level. (Especially common during pre-season when everyone's unusually un-fit :D )
What we mean by this in terms of external observable actions is that, when we get very tired we stop making good decisions, and our fine motor control drops - we chose bad passes, we take people on when we wouldn't normally do so when not tired, our hand-eye cooridnation drops so that we can no longer execute skills as well, etc. etc. etc.
Certainly the effect exist - those actions are clearly observable.
But is there truly a physiological effect behind it, and if so, is that effect truly about oxygen levels in the brain?
If one were able to install an O2 sensor in one of major arteries leading to the brain, would you be able to see actual drops in O2 levels, when someone is reaching a point of combined aerobic and anaerobic exhaustion (hockey is a heavy mix of both forms of activity, like many team sports)