According to Allan Fields, chief physician for the Florida Boxing Commission that also oversees MMA sanctioning:

Walker passed the "most strenuous tests that possibly you can impose on an athlete."

A stress test on Walker's heart given by a "major cardiac institution," Fields said, produced the highest score of anyone ever tested by facility.

"He's in as fine a shape as Muhammad Ali or any of these people we've had the care of," said Fields, a former U.S. Olympic team physician. "This guy is 47 going on 22, as far as his physical fitness goes."

Even so, Walker acknowledged there will be skeptics.

"There have been some athletes that have been totally an embarrassment," Walker said. "Jose Canseco, it's insulting, the guy never trained. I'm a guy that's serious about this. This is fighting, you get hurt.

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