How Matt Damon, 45, Bulked Up for His Highly Anticipated Return as 'Jason Bourne'

Universal Pictures

DAYUM, Matt Damon.

The Massachusetts native has come a long way since he first starred as Jason Bourne in The Bourne Identity when he was just 30 years old. In the era following beefcake Avengers and the muscle cars in The Fast & the Furious, the actor has turned Jason into a muscly hero of his own.

It's been nearly a decade since we've last seen Damon portray the now-iconic role of the CIA's most lethal former operative. He appeared in The Bourne Supremacy in 2004, followed by The Bourne Ultimatum in 2007, before Jeremy Renner took over the franchise in 2012. Now he's back (and arguably better than ever) in the franchise's latest installment, Jason Bourne, which hits theaters on Friday, July 29.

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From lean and toned at 30 to bulky and badass at age 45, getting into Jason Bourne shape for the fourth time was no easy feat. In fact, Damon told ET that even though reprising the role was "fantastic," transforming his physique was quite brutal.

"It's kind of arresting now," he revealed. "I remember in the third movie, seeing what I looked like in the first [one] and going, 'Oh yeah, I am getting a little older.' Now I go, 'My God, who is that kid?'"

Hot off the sets of The Martian and The Great Wall, Damon began working with trainer Jason Walsh, hitting the gym twice a day for 90-minute, high-intensity training sessions prior to filming Jason Bourne. His strict diet consisted of only vegetables and protein -- "no carbs."

"I like to eat. My favorite thing is a big bowl of pasta and a glass of red wine and my friends around," Damon said. "That's a great night."

"So, yeah, it was tough, but at the end of the day it was worth it," he continued. "[Director] Paul [Greengrass] said to me earlier, 'Look, if the movie starts and you look like you've been living well, we don't have a movie.'"

And Greengrass was right. Aside from Jason Bourne's famous action-packed scenes and amnesia-fueled flashbacks, what makes Bourne "Bourne" is his swoonworthy exterior.

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Over the course of the franchise, we've seen Damon's body transform tremendously for the thrilling role. In The Bourne Identity, we first caught a glimpse of his impeccable pecs as he was pulled out of the Mediterranean Sea following a failed mission. He was shirtless, and his lean, muscly back was on full display as he lay face-down on a table before reestablishing consciousness. We saw him do pull-ups on a boat. We saw him fight off cops from a park bench as he attempted to learn his identity. We saw him flawlessly climb down the wall of a building, Spider-Man style. We witnessed him nearly naked in a steamy make out session with Franke Potente (Marie).

Two years later, Damon upped up his physique for the second film, The Bourne Supremacy. Clad in an olive green cut-off tank top, a peek at his bulkier biceps was the perfect intro to the equally enticing thriller. Fast-forwarding three more years, The Bourne Ultimatum mostly featured Damon donning a black coat as he continued his search for information on his life pre-Operation Treadstone. It was clear he was just as ripped and badass, nonetheless.

Now, 2016's Jason Bourne sees Damon back in action, showing off the results of those intense gym sessions as he appears shirtless within the first five minutes of the film in a Fight Club-like scene. The beloved Bourne continues to battle his enemies and keep viewers at the edge of their seats with high-speed chases, nearly impossible missions and a long-awaited reunion with his former colleague, Nicky Parsons (Julia Stiles).

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Damon only has about 25 lines in the latest installment of the franchise, but when you have a bod like that to stare at, does it really matter? As Eric Byer (Edward Norton) so perfectly puts it in 2012's The Bourne Legacy -- the only Bourne film not to star Damon -- everyone's "publicly celebrating their top secret man crush" once again.

"I'm really grateful that I got to play this character," Damon said. "You know, it did so much for my life and my career. It really changed everything, so to be able to come back and do it again was really cool."

"It's been 16 years," he said about first taking on the role. "It's a huge chunk of my adult life, and I can't compare it to anything else to my career. It's really a wonderful thing to be able to come back and get together with these people and do this. It's just really great."

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