There is a rumor that CGI gave the actors the bodies they had in the movie, but that is not true.  There are videos of some of the training they went through in an intensive two month program.  Definitely check a few of them out!

Training for athletic performance is one of my interests, going back quite a few years.  I found this site and the focus of the training group phenomenal.  They get right down to it and produce functional strength.

I feel they got very close to the kind of physique that a Spartan warrior probably had.  There has been no soldier on the face of the earth trained better or more thoroughly than a Spartan, before or since.