American actor Sydney Sweeney transformed into former professional boxer Christy Martin for the biopic Christy, which chronicles her life in and outside the ring.

Directed and co-written by David Michod, the film tracks two decades of Martin’s life. During that time, she stumbled into the sport and, by virtue of her raw talent, turned into one of the most successful female boxers of the 1990s.

The film also portrays her relationship with her coach-turned-husband Jim Martin, played by Ben Foster, who takes control of her career and her life, with harrowing consequences.

Martin went on to found Christy’s Champs, a non-profit supporting family abuse victims, and telling her story came with responsibility, Sweeney said at the film’s premiere at the London Film Festival on Friday.

“Honestly, it’s a really heavy weight to carry,” she said. “I feel the importance of her story and what it means to other people. And she’s such a huge advocate, and she inspires me to also be an advocate as well.”

Known for her aggressive fighting style and powerful punches, Martin, now 57, was the first woman to sign with boxing promoter Don King and competed from 1989 to 2012. She was closely involved in the production and visited the set often during the film shoot.

Euphoria and Anyone But You star Sweeney spent some three months preparing for the role, working with a boxing coach, a weight trainer and a nutritionist.

“I put on 35 pounds (around 16 kg) and it was definitely a very intense physical transformation. But I felt so strong,” she said, adding that the fighting scenes in the film were very realistic. “I got a concussion. I might have broken someone else’s nose.”

The film celebrates Martin’s boxing achievements and also touches on themes of domestic violence and sexual identity. “We set out to make a film that’s about many things, not just boxing,” said co-writer Mirrah Foulkes.

“I realised there was a larger story to tell about how these kind of coercive-control relationships work, the one that she has in this film with her husband-slash-trainer,” added Michod. “I wanted to understand how they function, because they function all too commonly all over the world and often with horrific consequences.”

Christy begins its global cinematic rollout on November 7.