Psychologically it is a lot easier to keep on going when you are winning. Once you fall behind your hear drops and the same effort feels harder...
The other factor in that is Adrenaline..you don't always feel the tiredness at all till the effect subsides.
They dont have any other races in 4 years then ? Strangely enough I am certain I have seen Bolt run more than just at the Olympics !!
That's because the team gets on with it. You do not complain but it does not mean they are not tired. There is an old saying that if you are ill and tell people, then half do not want to know, and the other half are glad. So say nothing and just get on with it.