Sydney Sweeney gained 34 pounds and built a fighter's body

Beneath the glittering shops of Beverly Hills, beyond the valet stands and designer boutiques, lies an obscure underground gym with no signage, no website, and no social media. This is where Hollywood's most famous bodies are secretly built—Kumail Nanjiani's Marvel muscles, Hilary Swank's Million Dollar Baby-like strength, and most recently, Sydney Sweeney's transformation into boxing legend Christy Martin for the upcoming biopic 'Christy.'

Trainer Grant Roberts laughs and tells Yahoo, "This place is paparazzi-proof. You literally have to drive underground. It's like the Batcave. No one will know about me when the results come in."

For Roberts, this work was a full-circle moment. Twenty years after preparing Swank to play a fighter, he found himself training another powerful actress willing to push her body and ego to new heights. "The athleticism, the mental mindset, the passion—it was like turning back time," he says. "In that way, Sydney reminded me a lot of Hilary."

Still, Roberts says he approaches every new client, especially actors, with a bit of skepticism. "You never know what you're getting into," he says. "You think, 'Will they be receptive? Is this person a prima donna? Does she really have the discipline?'" When he met Sweeney over Zoom, all his doubts were immediately dispelled. "She was hesitant at first—quiet, attentive—but as soon as we started working, everything changed. She's the sweetest person you'll ever meet, and she was absolutely charming from day one."

Becoming Christy Martin

When Sweeney first agreed to play a real-life warrior—whose nickname was "Coal Miner's Daughter"—she knew she couldn't fake it. Over the course of 12 weeks, the 5-foot-3-inch beauty set out to gain more than 30 pounds and shape her body into a champion's shape.

"Think about Sydney's courage. She's known for her beauty," Roberts says. "She had to look like Christy Martin, and of course, Christy Martin is a 140-pound warrior—but without disrespecting Christy Martin—she's not known as a beauty queen. She's used to getting punched in the face, so Sydney said, 'Yeah, I'm going to gain 30 pounds, and people can punch me in the face.' That's a very bold move for a Hollywood star."

Roberts says that about two-thirds of Sweeney's weight was muscle. "She gained about 34 pounds," he says.

Her training camp wasn't anywhere near Beverly Hills. Sweeney and Roberts set up their training center in "Nowhere, Idaho" and created a makeshift training area with a heavy bag, a speed bag, a small weight room, and a ring they built themselves. "It couldn't have been more old-fashioned," Roberts says. "We were on a lake, somewhere remote. It was like an old-fashioned fight camp, and it was a lot of fun."

The first six weeks focused on building her body—not just size, but also strength. "The work was to make her look and move like a fighter," Roberts says. After laying the foundation, they brought in boxing coach Matt Baiamonte to choreograph her ring work.

Roberts adds, "Sydney's athletic ability blew me away. She's incredibly athletic and incredibly motivated."

Mind over Muscle

Roberts says that despite all this physical intensity, the real transformation came from Sweeney's mental focus. "The biggest challenge for her was learning to disconnect from the rest of the world," he says. "She's very hardworking, but when she commits to something, she's completely immersed in it. Every day was scheduled—training, choreography, nutrition, recovery. She never gave up. Never complained."

He recalls that they had only 11 to 12 weeks before production began—a blink of an eye by Hollywood standards. Roberts says, "Studios always want to do it fast. It's like a game: Can you have the product ready in nine weeks? Ten? Twelve? Sydney did it in about eleven weeks, and the result was incredible."

He says that after all, she wasn't pretending to be a fighter—she was a real fighter. "I had no doubts about Sydney from the start. And I can usually tell within the first few workouts how she responds, how flexible she is, and how willing she is to do more, or in some cases, less," he says. "But that wasn't the case with Sydney."

Weight Loss

After filming was completed, Roberts also helped Sweeney get back to her natural shape—a process he believes is just as important as her size. "People think losing weight is easy, but it requires just as much discipline as gaining weight," he says.

Sweeney's days of PB&J, Chicken-fil-A, and milkshakes are over. When traveling after filming,