“You’ve got to drive the body to the last inch of energy then go on! You gain nothing by just going up to where your body says you’re tired. The body will build and grow only to fit the demands the mind makes upon the lazy body. If all you do is exercise until the body is tired, the body will get lazy and stop a bit shorter every time. You must go to the point of exhaustion, then go on.
That way the body figures out, “We’ve got to build up more strength if that crazy mind is going to drive this hard!?” If you always quit when you are merely tired, you will never gain. Once you let the body tell the mind when to quit, you are whipped for sure. You can not gain by listening to the body. We can become much stronger if we drive the body. We use about one-tenth of the available strength of our bodies and less than that of our minds.
-General George Patton