During intense exercise:
[Physiologist Tim Noakes] proposed that the brain is wired to protect itself by pre-emptively shutting down your muscles before any part of your body reaches total failure. If your muscles are being depleted of oxygen and your heart is working too hard; or if you are becoming dangerously dehydrated; or if your core temperature is rising excessively; or if you are climbing a mountain and the amount of oxygen reaching your brain drops significantly — in all of these situations, a "central governor" in your brain acts to slow you down or stop you before you do irreversible damage. You stop not because you can’t physically go any farther, but because your brain thinks you shouldn’t.
Some ultra-endurance athletes can override this.
The Dish 