The naturally slim star had to get buff fast when he was cast as the hero in Prince of Persia: The Sands of Time. And he did it in quite an unusual way, by free running. Also known as Parkour, free running involves leaping, lunging and bouncing through the urban environment.
"We worked really closely with David Belle, who invented Parkour," said Gyllenhaal at the time. "I worked with a lot of gymnasts. I started jumping off of a lot of things that were padded and learned the fundamental stuff, and then slowly started working on harder surfaces, really carefully. And I'd say, "Ok, I'll try it, and then I'd say, OH GOD. OH, please.But then I'd get it, and we'd go a little further and a little longer and a little higher and just kept it up."
So there are ten men who went from skinny or chubby to super-fit, and all in different ways. Though, as ever, taking exercise and eating sensibly were at the heart of these dramatic physical transformations.