Robert Downey Jr. Puts on Muscle for Iron Man 2

Iron Man 2, starring Robert Downey Jr., hits the big screen today -- and we're willing to bet that at least one movie-goer will be buying a ticket to the action film for the chance to get a peek at Downey's muscular, well-toned upper body.

"When we did Iron Man 1, we had five months before the shoot to get ready, so we used more traditional, heavy-weight training," Downey Jr.'s trainer, Brad Bose, told UsMagazine.com. But this time around, the actor, 45, had just wrapped Sherlock Holmes and was down to 151 pounds when he began shooting the action movie.

Downey Jr.'s intense routine to get him bulked up included what Bose refers to as "Rocky IV meets high-tech training," which brought him to 175 pounds in the first three to four weeks of filming.

Part of Downey Jr.'s weight training included pushing a custom-built wheelbarrow with 600 to 700 pound weight stacks around an obstacle course; beating car tires with sledgehammers (up to 20 pounds in each hand); and dragging fire hoses filled with sand and water to build up shoulder muscles and pecs. Bose trained with the Iron Man for about an hour-and-a-half at least three days a week, while Downey Jr. trained in kung fu two to three days with Eric Oram.

Of course, Downey Jr. isn't the first celebrity who's been required to quickly bulk up (or slim down) for a film. For his role in "Prince of Persia," Jake Gyllenhaal worked out two to three times a day, six days a week, and followed that routine for three to four months, in addition to stunt training for the film. Christian Bale shed pounds for his role in "The Machinist" by eating primarily apples and drinking coffee. On the flip side, stars like Renee Zellweger packed on the pounds for the "Bridget Jones" movies, while Robert de Niro added 60 pounds for his 1980 role in "Raging Bull."

"It's not healthy," said Sylwia Wiesenberg, an AFTA -certified trainer, former Polish gymnast and founder of Tonique Fitness. "It puts a strain on your heart, especially if your body isn't ready. In order to gain bulk this fast, many celebrities end up on a short program that packs on heavy, heavy weight, and works the muscles to an extreme in order to get them to grow." The main issue, said Wiesenberg, is that many celebrities immediately stop this intense training and rapid muscle-building right after they are finished with a film.

Her recommendation? Continue working out on a daily basis. "I realize many people, especially celebrities, don't have hours a day to work out when they are not training for a particular role," she said. "But still, if you are working out intensely for two months, gradually decrease the amount of time each day; don't just stop working out altogether. This way, your body won't go through that rapid change of size -- yo-yoing from extra strong to flabby back to extra strong again isn't good. For the average person it is much better to follow more gradual and sustainable program that incorporates exercises into lifestyle on daily basis."

At least one star is aware of the toll that changing shape has on her body; find out who is unhappy about having to yo-yo diet.