In the new movie Battle of the Sexes, premiering September 22, 2017, Emma Stone plays tennis legend Billie Jean King. While we know from Instagram that Stone works out and can deadlift like a champ, this is the first time the actress has portrayed an athlete on screen. Unsurprisingly, it took a lot of hard work in the gym for her to prep for this role.

Stone's trainer, Jason Walsh, C.S.C.S., NSCA-certified personal trainer and founder of Rise Movement and Rise Nation in Los Angeles, tells SELF that Stone came to him for a workout plan when she decided to take on the role. "When I got word from her that she got this role and heard what she wanted to look like and feel like, we sat down and formulated exactly how to get to that point," he says.

"She knew she'd be playing one of the greatest athletes ever. She took it really seriously," Walsh adds. Still, Stone is just as goofy in the gym as you'd imagine. "She’s really really good at breaking me with laughter," Walsh says. "She’s an asshole—she knows exactly how to get out of a move or slow the momentum down when she wants, and it’s not by procrastinating, it’s strictly by pure humor."

Here's what Stone did to physically prepare for her epic role.

Her original goal was to add 10 pounds of muscle in three months. By the end of her training, Stone gained 15 pounds of muscle.

"We had three months to get her to look and feel like an athlete," Walsh says. "We started with the goal of putting on at least 10 pounds, and then when we got there, we decided to put on another 5 to just have that physique as close as possible to Billy Jean."

As she progressed, she loved noticing that she was stronger. Walsh says that once she had that "feeling of moving something that was outside of the scope of what she thought she could do, then she was like, 'Hell yeah, I can't believe my body’s doing this!' " While gaining 15 pounds of muscle is a lot, especially for someone who doesn't typically work out to build bigger muscles, Walsh says Stone was thrilled about her progress. "She smiled ear to ear," when she reached her goal, he says.

Melinda Sue Gordon/Fox Searchlight

Emma Stone stars with Steve Carrell in the upcoming "Battle of the Sexes," where Stone plays tennis legend Billie Jean King.

It was especially important for Stone to stay injury-free because filming would probably be delayed if she got hurt.

"I can't tell you how many actors kill themselves to look a certain way and everything else as far as resiliency and functionality and being healthy is thrown to the wayside, and that doesn’t make sense to me," Walsh says. "The science of training today is meant to keep athletes healthy, keep them on the field."

He says he thinks of Emma (and any actor) as a pro athlete—if she gets injured and has to sit on the bench, filming stops. "That’s a lot of pressure on a trainer’s shoulders," Walsh says. "She came as close to looking and feeling like an athlete as possible, and the thing I'm most proud of is that she didn’t get injured."

An important factor here is that Stone started training a few months out. Walsh says he's been asked to train actors four or six weeks before filming, and has turned it down because he knows it's not the best way to reach a goal and avoid injury. "I’m not going to take on that responsibility if we don’t have time to do it."

Stone's training sometimes called for two workouts per day.

Since Stone already had a good base strength (she trains with Walsh regularly), her workouts built on that foundation. Walsh simply had to increase the volume of her workouts—by increasing both weight and reps—to help her build more muscle. Yes, some days that meant two workouts per day—an a.m. session and a p.m. session.

While Walsh switched up the exact workouts each day, overall, Stone's program included a lot of basic strength moves like sled pushes/pulls, hip thrusts, pull-ups, and push-ups. He says she was also deadlifting 185 pounds, front squatting 100 pounds, lunging with 70 pounds, and doing single-leg squats with 100 pounds.

Melinda Sue Gordon/Fox Searchlight

Actress Emma Stone spent three months in the gym to build the muscle and fitness she would need to play tennis star Billie Jean King in "Battle of the Sexes."

The other part of her muscle-gain formula? Consuming more calories.

"If you don’t eat enough, I don't care how hard you work out, you’re not going get those gains," says Walsh. "We increased [Stone's] caloric intake to approximately 2,500 to 3,000 calories a day." Instead of having her focus on calorie goals, though, Walsh just gave her directions on the type of things she should be eating more of and how much. His basic recommendation? At each meal, eat as many veggies as you want, some sort of protein, and healthy fats.

She also relied on high-calorie shakes right after each workout to supplement her caloric intake. Stone's post-workout shake—which Walsh gives to other clients, too—included whey protein, Udo's oil (a mixture of flaxseed oil, sesame seed oil, and other fats), ashwagandha (an herb commonly used in Ayurvedic medicine), a handful of spinach, and nut milk or water.

Now that filming is over, Stone will still keep at it—but definitely dial back on that intensity.

Stone's hard work got her to her goal of looking more like Billie Jean, but Walsh notes that this type of transformation changes you mentally as well. "It’s a huge psychological advantage to feel really comfortable in your own skin. I think it helps even with the character," he says. There's definitely something to be said for being able to look and feel the part, inside and out.

"It makes me very very happy and proud when someone puts themselves through something like that and balances it out well enough and I educate them, and they want to keep going and training after they’re done with the actual movie," Walsh says.

Moving forward, Walsh says they'll keep at the strength workouts, but dial back the volume to probably just three times a week—which is more on par with what they did before Stone started training for Battle of the Sexes. Three strength-training sessions per week leaves room for Stone to do the other things she enjoys, like hiking, Rise Nation, indoor cycling, and yoga, according to Walsh. "I don’t want my clients to spend all their time in the gym, I encourage them to go out and do other things." He tells his clients to take a break when he thinks they're pushing too much, because that balance is so important to him. Strength, he notes, should be the foundation you always come back to and that helps keep your body ready to do the other stuff you love.

Sounds like Stone's got that whole foundation thing handled.

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