- - Sydney Sweeney gained 34 pounds and built a fighter’s body for 'Christy.' Her trainer reveals how she did it.

Taryn RyderNovember 7, 2025 at 9:00 PM

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Sydney Sweeney's trainer, Grant Roberts, reveals how he helped the actress transform her body for Christy. (Photo illustration: Julia Meslener/Yahoo News; photos: Steven Simione/WireImage via Getty Images, Courtesy of TIFF)

Somewhere beneath the gleaming storefronts of Beverly Hills, past valet stands and designer boutiques, sits a nondescript underground gym with no signs, no website and no social media. It’s where Hollywood’s most famous bodies have been built in secret — Kumail Nanjiani’s Marvel muscles, Hilary Swank’s Million Dollar Baby strength and, most recently, Sydney Sweeney’s transformation into boxing legend Christy Martin for the upcoming biopic Christy.

“This place is paparazzi-proof,” trainer Grant Roberts tells Yahoo with a laugh. “You literally have to drive underground. It’s like the Batcave. The only way anyone hears about me is through results.”

For Roberts, the job was a full-circle moment. Twenty years after helping Swank bulk up to play a fighter, he found himself training another powerhouse actress willing to push her body — and ego — to new extremes. “The athleticism, the mental mindset, the drive — it was like winding back the clock,” he says. “Sydney reminded me so much of Hilary in that way.”

Still, Roberts says he approaches every new client, especially actors, with a hint of skepticism. “You never really know what you’re walking into,” he says. “You wonder, are they going to be receptive? Is this person a prima donna? Do they really have the discipline?” When he met Sweeney over Zoom, any doubts he might have had disappeared quickly. “She was reserved at first — quiet, observant — but that changed the minute we started working. She’s the loveliest person you’ll ever meet, and she was all in from day one.”

Becoming Christy Martin

When Sweeney first signed on to play the real-life fighter — nicknamed “The Coal Miner’s Daughter” — she knew she couldn’t fake it. Over the course of 12 weeks, the petite, 5-foot-3-inch actress committed to gaining more than 30 pounds and reshaping her body into the body of a champion.

“Imagine the courage that Sydney had. She’s clearly known for her beauty,” Roberts says. “She had to look like Christy Martin, and of course, Christy Martin is a walk-around 140-pound fighter — but no offense to Christy Martin — not known to be a beauty queen. She’s used to getting punched in the face, so Sydney’s like, 'Yeah, I’m going to put on 30 pounds, and people can punch me in the face.' That’s a pretty brave move for a Hollywood starlet.”

Roberts says about two-thirds of what Sweeney put on was muscle. “It was somewhere around the 34-pound mark that she ended up putting on,” he says.

Their training camp was nowhere close to Beverly Hills. Sweeney and Roberts set up shop in the "middle of nowhere, Idaho," and created a makeshift training area complete with heavy bags, a speed bag, a tiny weight room and a ring they built themselves. “It couldn’t have been more old school,” Roberts says. “We were right on a lake, in the middle of nowhere. It was like the old fight camps, and it was so much fun.”

The first six weeks were focused on building her body — not just size, but power. “The job was to make her look and move like a fighter,” Roberts says. He brought in boxing coach Matt Baiamonte to choreograph her ring work once the foundation was set.

“Sydney’s athleticism blew me away,” Roberts adds. “She’s incredibly athletic and incredibly driven.”

Christy Martin and Sydney Sweeney attend the premiere of Christy on Oct. 25 in Hollywood. (Photo: Axelle/Bauer-Griffin/FilmMagic) (Axelle/Bauer-Griffin via Getty Images)The science (and snacks) behind the strength

In addition to daily workouts — often twice a day — Roberts handled all of Sweeney’s diet himself.

“I play the role of nutritionist with every client. For Syd, it was the other 20 hours of the day,” he says, because Sweeney typically did double training sessions. Each program began with a 3D body scan and a full-body composition analysis to tailor macros, meal timing and training intensity. For Christy, the goal wasn’t a sculpted, superhero look. It was believable power.

That meant more food, not less, and yes, some of it was comfort food, as the Euphoria star indulged on lots of PB&Js. “I didn’t tell her to eat [that] ... but once she met her protein goals, she had freedom to eat less nutritious foods for the fat gain objective, to look like the character.”

Sweeney started each morning with protein shakes — "We did a lot of shakes" — and Roberts says protein was the foundation of "every single meal." Before bed, she added a shake with casein, a slow-digesting protein, to support overnight recovery.

“Timing is everything,” Roberts says. “It’s not just how much you eat, it’s when you eat it. Every element of Sydney’s day — training, meals, recovery — was precisely timed.”

Around week five of training, Sweeney had already gained 20 pounds. “I have a video of [her stepping on the scale],” Roberts says. “Her face, I couldn’t tell if it was joy or fear, but she was just shocked to see the needle on that scale go up so high. She had dense shoulders, and she was developing her back. We put 4 inches on her glutes alone, and she was loving that part of it.”

Grant Roberts attends a screening of Christy during AFI FEST 2025. (Photo: Michael Kovac/Getty Images for AFI) (Michael Kovac via Getty Images)Mind over muscle

For all the physical intensity, Roberts says the real transformation came from Sweeney’s mental focus. “The biggest challenge for her was learning to shut out the rest of the world,” he says. “She’s in high demand, but when she commits, she’s all in. Every day was scheduled — training, choreography, nutrition, recovery. She never quit. Never complained.”

He recalls that they had just 11 to 12 weeks before production began — a “blink” by Hollywood standards. “Studios always want it faster,” Roberts says. “It’s like a game: Can you deliver the product in nine weeks? Ten? Twelve? Sydney did it in about eleven, and the result was unbelievable.”

By the end, he says, she wasn’t pretending to be a fighter — she was one. “I never had any doubt with Sydney right from the get-go. And I can tell those things usually within the first one or two workouts, how they respond to it, their resilience and their desire to do more, or in some cases, less," he says. "But that was not the case with Sydney."

The weigh back

When filming wrapped, Roberts also helped Sweeney transition back to her natural size — a process he says is as important as the build itself. “People assume going back down is easy,” he says, “but it takes as much discipline as putting the weight on.”

Gone were Sweeney's days of indulging in PB&Js, Chick-fil-A and milkshakes. While traveling after filming wrapped, Sweeney restricted her diet, something Roberts says isn’t his preferred method for every actor, but ultimately balanced out well for her, thanks to her strong foundation.

Roberts wants women, especially, to understand that strength training isn’t the enemy. “There’s this fear — ‘I don’t want to get bulky or masculine,’” he says. “But nothing could be further from the truth. Weight training creates shape. It’s how you get rounder glutes, shapelier arms, stronger posture. Cardio will never do that.”

Sydney Sweeney at Variety's 2025 Power of Women: Los Angeles event on Oct. 29, 2025. (Photo: Jordan Strauss/Invision/AP) (Jordan Strauss/Invision/AP)

In fact, Roberts believes Sweeney is proof of that, and you can still see the results of all that hard work.

“If you take a close look at Sydney Sweeney today vs. Sydney Sweeney pre-Christy, you’ll see nicer, rounder shoulders, a little shapelier arms, a little rounder butt," he says. "All of those things were the benefit of what we accomplished.”

Twenty years after Roberts helped Swank win an Oscar for Million Dollar Baby, he’s watching a new contender step into the ring. Early awards buzz suggests Sweeney’s performance could be the next to earn Hollywood’s attention.

When Christy lands in theaters on Nov. 7, audiences will see more than a body transformed. They’ll see an actress who built one — punch by punch, meal by meal, rep by rep.

Original Article on Source

Source: “AOL Entertainment”