C. Molly Smith
Updated September 20, 2017 10:42 am

We were fully aware that Emma Stone got buff to play tennis pro and social activist Billie Jean King for Battle of the Sexes, a film that centers on the 1973 woman vs. man-type match between King and Bobby Riggs. What we didn’t know is exactly how Stone got so ripped.

Well, Stone’s Battle of the Sexes co-star Natalie Morales just provided some pretty hilarious insight.

Morales plays Rosie Casals in the film, King’s fellow tennis player and activist who was a commentator during the infamous match. As Morales recalled to HelloGiggles, Stone’s muscles were a big part of what made the Oscar-winning actress so believable as King.

Whoa, indeed.

Melinda Sue Gordon / Twentieth Century Fox

Morales went on to say that a lot of laughs were shared on the Battle of the Sexes set.

And Stone would participate in all of that — while doing push-ups simultaneously.

Not only that, but Stone put a huge emphasis on precision, so she basically choreographed King’s tennis moves to get them down just right.

Morales added, “She watched Billie Jean’s performance and learned her moves like a dance, and then would basically dance with a tennis ball. That’s how she did it. It’s really awesome to see because I see all her work, but it just looks like Billie Jean. Emma deserves everything good that’s coming to her.”

See Stone’s sick tennis moves — and equally awesome biceps — when Battle of the Sexes lands in theaters on September 22nd.