The 34-year-old actor underwent extremely rigorous training to prepare to play Billy Hope, a boxing champion who's attempting to get his life back on track—but the tough regimen didn't just transform him physically.
Though he obviously did that as well!
Gyllenhaal went from a relatively slight 147 pounds to a ripped 175 pounds in just six months, trainer Terry Claybon, who whipped Jake into shape for Southpaw, in theaters July 24, tells E! News. And they had a lot of work to do when they first met up to discuss training.
"He wasn't in any shape at all," Claybon said. "We met at a boxing gym, and he was jumping rope. Just the way he was jumping rope, I knew we had a lot of work to do."
Claybon didn't let Gyllenhaal cut any corners: "Everything we did in training was as if he was having a professional boxing match."
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"The most important thing we worked with Jake on was his defense, because if a fighter has good defense then he can protect himself," Claybon continued. "Gyllenhaal didn't have any days off, training seven days a week for six months. "We were working out three hours in the a.m., three hours in the p.m., six hours total a day, seven days a week."
The regimen included at least "2,000 sit-ups a day, push-ups, dips, flipping 350 pound tires, and running eight miles five days a week."
The first fight scene began with Jake nervous and weary of his skills.
Claybon recalls, "I was wrapping his hands and getting prepared him prepared to go out for his first fight scene, and he kept complaining about his hands...He kept complaining, because he was nervous. Until, I really just got frustrated, and just put the gloves on him, and told him we had to go, because we were already 30 minutes behind and they were waiting on us.
"But I knew he was nervous. Once we got out there, after the first take, I couldn't get him back to the dressing room!"
As Jake physically transformed, he began changing internally as well (and we don't mean the occasional pulled muscle).
He learned how to deal with pressure better, and he built up his confidence, Claybon says. "I've seen him transform where he knew how to deal with pressure. From the first fight scene he was going out there in front of a live audience of 2,000 people. Going out there for your first fight scene is like going out for your first fight."
"Jake really built up his confidence with this movie," the trainer adds. In fact, "he wouldn't allow his stunt double to do anything. He did everything. His stunt double never worked."
Gyllenhaal didn't train alone, either! Director Antoine Fuqua was training with him every day.
And co-star Rachel McAdams was so inspired by Gyllenhaal's transformation that she too joined in, and decided to start training three days a week "to see what her 'husband' was going through," Claybon explained, referring to the actors' onscreen relationship.
McAdams and Gyllenhaal didn't train together, since their regimens were different, but sometimes they'd run into each other, and would "watch each other work out hard." Claybon says, "She can box! She's got a good right hand, I'm telling you."
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Claybon agrees that Gyllenhaal was a changed man both inside and out after Southpaw.
He tells E! News, "I think that after this movie Jake's about to be one of the biggest stars out there, because of the strength. He's grown a lot. To pull off this boxing movie, he had to put everything into it. He had to put his heart and soul into this."
And while this role required him to leave everything in the ring, we're betting that Jake's heart and soul will be as beefed up as ever in his next film.