From the review:
Spock and Kirk defeat the evil Khan (Ricardo Montalban and his fake chest, both fan favorites)
Both Nick Meyer and Leonard Nimoy have gone on record to confirm that was indeed Montalban’s chest.
When asked about it:
director Nicholas Meyer replied that even in his sixties Montalbán was "one strong cookie" and that his real chest was seen on film; Khan's costume was specifically designed to display Montalbán's physique.
I don’t get it, why is it so difficult for some people to accept a 62 year old man could be in excellent shape? Besides, it's pretty obvious the costume was designed to make it more impressive. If it wasn't a real chest, I can't imagine why they'd call such attention to it.