Too tired to be tense

There's a point you can reach in a hard karate class where your muscles are still strong enough to hold you up, but there's no spare energy. In that state, you will sometimes find that your techniques are faster and you're moving more easily than before. Because your muscles are tired now, they're not fighting against your own movements. That's the kind of relaxation that you need to move quickly.

I think I've reached a similar emotional state once or twice in my life. My mind gets too tired to put up a fight any more, and suddenly I felt more relaxed. It's a very odd feeling.

Mokalus of Borg

PS - If you keep going, you lose the advantage and just end up tired.
PPS - I expect the same is true of the mental version.