[n4te]

The only thing that makes sense to me is that the body gets more efficient.

[jay_kyburz]

That what I imagined as I was reading, the act of breathing and pushing blood around the body just becomes easier when you are fit. (requires less energy)

[Workaccount2]

Your nervous system also becomes more efficient at zapping the right muscles at the right time. The effect of this can be pretty dramatic too. Even a slightly out of sync orchestra just sounds wrong.