Sadly, Muhammad Ali was showing the symptoms of Parkinsons disease while he was still fighting. All the "experts" put it down to something called "punch drunkedness" and ignored the severity of it.
Mike Tyson has spoken about Ali and his frame of mind in the ring. Actually broke down in tears describing just how different of a cat he was. Most are as tough as they get in this world but only a few are prepared to die in the ring and die for their beliefs. Ali was one of those people (LaMotta another).
Just kept fighting for years on end when he should have retired long before.