Perhaps fighters are just trying to stay confident. But each time I see a fighter interviewed and asked about it, they either have said it isn't real or it is hardly a factor.
They are trying to stay confident. Ask anyone who's ever fought. If you haven't fought in a while there is definitely rust. It's the same with any other skill. If you don't use it you get worse at it.
That is not dead which can eternal lie and withs strange aeons even death may die.