I only know this Chuck Norris anecdote third hand so I might have some details wrong, but several friends of mine were deeply into Shotokan Karate and told me about it. Tsutomu Ohshima, the founder of American Shotokan, used to hold occasional Special Training camps where ultra serious practitioners could spend multiple days doing particularly arduous training. Chuck Norris was apparently invited to one of these camps back in the 70s and participated with his usual full intensity. In one session participants were doing some sort of grueling endurance workout (unfortunately I forget which) and one by one the black belts started dropping out as they got tired. Not Chuck Norris. He never quit, but kept going until he passed out from exhaustion. Master Ohshima was very impressed by this level of mental toughness and thereafter always thought Chuck Norris was the real deal. That's good enough for me.