Constantly sparred extremely hard in an attempt to improve durability? Some fighters avoid being hit with even decent shots during sparring... Have there ever been any fighters that did the exact opposite? McCall springs to mind considering how he was Tyson's most consistent sparring partner.
Well, I suppose Joe Louis inadvertently toughened up in some sparring sessions when 'Chappie' Blackburn used to tie his right hand behind his back to develop his left. Apparently, Joe got beat up quite a bit in sparring before developing his left hand. But I can't think of anyone who deliberately got beat up in sparring, although I'm sure a few crazy fighters tried it.
Ali comes to mind. Little surprised that he was deteriorating fast past age 32, despite a 3 year-lay off.
Hurricane Ruben Carter, a natural middleweight agreed to mix it with Sonny Liston in a hard fought gym match. The story goes Carter managed to make it out of the first round. After the round was over Cater retired with a busted nose and face, but he did prove his machismo amongst the boxing community. I cant think of any other guy giving up 50+ pounds willing to go toe to toe for free with a guy like Liston.
Dempsey beat up on his sparring partners as if they were speed bags. Some of them sustained similar injuries to Willard in his title fight. Louis had trouble finding people willing to sparr with him because at the end of the day he was only offering them money to take all that punishment.
Holmes put a $10,000 bounty on his head. If you could knock him on his butt, he would pay you $10,000. No one ever knocked him down. He wanted them to try their very best and to be sure that they truly nailed him if they could.
I was just thinking about that one. Ali just put his hands on the ropes and tried to make it look like nothing. Dokes was 18 or 19 and a rising prospect. Ali showed that he had balls of steel, but knowing how he ended up, it's kind of hard to watch him just takes punches for the heck of giving a show.
Getting used to taking punchers does improve punch resistance. Getting knocked unconscious reduces punch resistance