Sometimes, Walker doesn't have an appetite and will go through seven hours of wrestling, kickboxing, sparring and practicing jujitsu without having eaten for three or four days.

"It's just unbelievable," said Mendez, who trains Walker at the American Kickboxing Academy in San Jose, California. "He shouldn't be able to do what he's doing. I don't think it's possible to eat as little as possible and work out the way he does. There's no way. He's an unbelievable athlete."

Mendez doesn't try to change it either.

"You can't fix it, because it's not broken," he said. "You can try to understand it -- good luck with that."

And no, Mendez thinks it probably won't work for most people.

At 5:30 a.m., Walker wakes up to do 750 to 1,500 push-ups and about 2,000 sit-ups.