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Jake Gylennhaal, Southpaw

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If you haven't watched the trailer for Jake Gyllenhaal's upcoming film Southpaw, then you're missing out on some solid eye candy.

The 34-year-old actor seriously beefed up his buff bod for his role as Junior Middleweight Champion Billy "The Great" Hope, packing on 15 pounds of muscle in addition to training with professional boxers. 

Even more impressive is the fact that Gyllenhaal was fresh off the film Nightcrawler, the Dan Gilroy-directed flick for which the good-looking stud dropped a whopping 30 pounds (following the film, he returned to his normal weight before adding an additional 15 pounds of muscle mass). 

"I wanted Jake to learn how to box so he wouldn't be out there on the set trying to act like a boxer," Gyllenhaal's trainer and former professional boxer Terry Claybon tells Yahoo Movies. "I pushed Jake to the limit." 

WATCH: Check out the latest Southpaw trailer!

Jake Gyllenhaal

Frazer Harrison/Getty Images

According to Claybon, Gyllenhaal initially began working out three hours a day—an hour and a half in the morning and an hour and a half in the evening.

The silver lining? He was not required to stick to a very strict diet, thanks to his rigorous fitness regime, and was able to enjoy carbs in the morning as well as protein in the evening. 

After two months, Gyllenhaal's workouts bumped up to six hours per day and consisted of three hours of boxing in the morning, followed by three hours of strengthening, conditioning, and cardio at night. Additionally, he would do 1,000 sit-ups in the morning and then another 1000 at night, slowly increasing his endurance. 

"Gradually we built up, day-by-day, to 2,000. It takes time to do that," Claybon said. 

PHOTOS: Stars snapped on-set of their upcoming movies 

Jake Gyllenhaal, Southpaw

Weinstein Company

Gyllenhaal was unfamiliar with the sport until he began working with Claydon and has now developed a love for the craft. 

"I was blown away over how much work I had to do because of his rhythm, coordination, and timing," the 52-year-old trainer recalled of the first time he met Jake. "Every day, he was eager to learn more and more about the sport." 

Additionally, Gyllenhaal attended a few pro boxing matches to delve deeper into the mindset of his character. "That way, Jake can get out there and see how the audience has an effect on fighters," he explained. 

According to Claybon, the actor has four fight scenes in Southpaw, three with professional boxers, and while there's no fist-to-fist contact on set, the fights mirror a real match and were filmed in front of a live-audience. "When you're on a set, it's still important to look good and learn the choreography," Claybon said. "If you make a mistake, you're gonna get hit!"

What do you think of Gyllenhaal's impressive transformation? Tell us in the comments!

PHOTOS: More male celebs show off their surprisingly buff bodies