Arnold Schwarzenegger is a huge advocate of this; it's the power of one's mind. 

In exercise and nutrition you have two choices; to stop at what's perceived as good enough or to go all the way, to take it to the next level, to push yourself, and excel to your fullest potential.  

It is not the muscle's own temporary weakness that reduces performance, they find, but instead the brain places an unconscious 'brake' on the central motor drive to the limbs and therefore regulates exercise performance.

From Arnold's The New Encyclopedia of Modern Bodybuilding, "The mind is a dynamo, a source of vital energy.  That energy can be negative and work against you, or you can harness it to give yourself unbelievable workouts and build a physique that lives up to your wildest expectations."
 
Your mind is in control of your body and whether you achieve your goals (or not). 

To change your body's level of fitness, you must change what you are doing with it.  If you want to change your body's composition, you must change what you put into it.  In other words, if you've always done it, don't do it and if you've always eaten it, don't eat it.  Never succumb to weakness or routines.

BTW, the Arnold Classic is coming up February 29 - March 2nd in Columbus, Ohio.  It's an exciting weekend of bodybuilding, fitness, 12 Olympic sports, 17,000 athletes, educational seminars, and much more.  

Y'all going?