Jake Gyllenhaal is opening up about the intense workout and training required to transform himself into a boxing champion for his upcoming film Southpaw.

Jake Gyllenhaal Workout

When photos of Gyllenhaal as Billy ‘The Great’ Hope from Southpaw first emerged in December of 2014, audiences were stunned at Gyllenhaal’s bulky new physique. At the time, Southpaw director Antoine Fuqua revealed that Gyllenhaal had trained in Floyd Mayweather’s gym to attain his boxer’s body, saying, “We literally turned him into a beast.” Fuqua even suggested Gyllenhaal’s complete lifestyle overhaul led to a breakup.

The transformation is even more impressive considering he had just finished losing 30 pounds to play Louis Bloom in Nightcrawler when he was beginning to train for Southpaw. In Nightcrawler, Gyllenhaal appears thin and gaunt, a stark contrast to his boxer’s body in Southpaw.

As the release date for Southpaw looms closer, more details are coming out about Gyllenhaal’s impressive transformation. Former professional boxer Terry Claybon recently opened up about training the Academy Award-nominated actor. “I wanted Jake to learn how to box so he wouldn’t be out there on the set trying to act like a boxer. I pushed Jake to the limit,” Claybon told Yahoo Movies.

Claybon revealed that he made sure Gyllenhaal got in the habit of eating healthy means of carbs and protein, to ensure he had energy to burn during workouts. What began as three hours a day in the gym became six – “three hours of boxing in the morning and three hours of strengthening, conditioning and cardio at night.” By the end of their training period, Gyllenhaal was doing 2,000 sit-ups a day.

In addition to the one-on-one workouts, Claybon also threw Gyllenhaal into the ring to fight against trained fighters and took him to see professional fights. Claybon was also on hand during filming, helping Gyllenhaal with the fight choreography and making sure he kept his training up, even going a few rounds during their lunch breaks. On screen, Claybon said proudly, “You see a guy who knows how to box.”

When asked about the physical work it took him to gain 15 pounds of muscle for his role in Southpaw, Gyllenhaal said that one of the things he worked on the most was discovering the mentality of a fighter.

“There’s a tremendous physical aspect about getting into shape or learning how to box, but for me, you can’t play a boxer and just look like a boxer… To play a boxer, to play any role, you have to believe that you can exist in that world, or else your confidence in the scenes and your interactions with every character – particularly in a movie like this where you’re working with real fighters – is a waste of time,” Gyllenhaal said.

Southpaw hits theaters July 24, 2015.