Emma Stone has never tried to pretend she loves the gym. “I had a trainer during Spider-Man, and I discovered I have deep-seated rage when I'm holding heavy weights over my head," the Oscar winner said in a 2012 interview. "I'm usually a pretty peaceful person, but for some reason when I get in the gym, something bubbles up in me."

Relatable, but when you’re cast as Billie Jean King, a 39-time Grand Slam-winning tennis player, in Battle of the Sexes—about the famed 1973 match between 29-year-old King and 55-year-old Bobby Rigg—you can’t just rely on your acting skills; you need tennis skills to back them up.

emma stone Image: Fox Searchlight
To channel King, Stone put on 15 pounds (yes, 15!) of lean muscle, according to Jason Walsh, Stone's personal trainer and founder of Rise Nation. “The initial goal was 10, then we went for more because it looked and felt right,” Walsh explains. “She’s portraying one of the best athletes in history, so this weight gave her the tools she needed to fulfill that role, including the psychological side of it.”

Stone and Walsh worked out twice a day, doing major strength training exercises like 200-pound sled pushes, weighted lunges, and stability-challenging exercises like Bulgarian split squats and single-leg deadlifts. “Single-leg exercises helped Emma achieve stability around the joints, which allowed her to move and transfer weight like an athlete while lowering her injury potential,” Walsh says. Tennis players especially need that kind of stability, since their sport calls for rapid movements and high-impact shots that can do serious damage to the joints if they aren’t trained properly.

Because Stone is naturally lean, she and Walsh doubled her ratio of strength training to cardio while prepping for Battle of the Sexes. “Too much additional cardio would impede her progress of adding weight and muscle,” explains Walsh, who has also worked with actresses Minka Kelly and Alison Brie.

But since tennis calls for speed and agility in addition to strength, Walsh had Stone add 30-minute sessions on the VersaClimber machine during the last four weeks of her training. “Cardio interval training adds in a little extra full-body conditioning and fat burning without overworking the body,” Walsh says. Plus, switching up your workout is a surefire way to see real gains, since it keeps your muscles from getting used to any one type of exercise.

Stone will show off her skills on in the court when Battle of the Sexes hits theaters on September 22. Check out the official trailer below.

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