I think it's not the training per se. It's the age factor largely. Age affects training quality especially if boxer is not focused. And talking about focus, i think barrera didn't really think juarez could pull an upset off. Mental attitude is crucial in boxing you know, and we know that what barrera really wants is a rematch with pacman. It's the one thing he so desires before he really hangs up his gloves. That rematch would not only give him the megabucks (i wonder how much he made in the juarez fight) but also the glory of an exit win or lose. If he wins against manny, that would be the most colorful decoration to his legendary career. If he loses, he would say that he lost to the legit and real successor, a loss that's not hard for him to accept. Look at this article.

http://thepostman.prepys.com/archive...barreras-belt/