Aiden Monaghan/ Focus Features Alexander Skarsgård in The Northman

Between the eight-pack abs and ripped biceps, Alexander Skarsgård looks like someone you wouldn't want to mess with in The Northman, the actor's new film in which he plays a Nordic Viking trying to avenge his father's death. But his shredded body took serious work, including intense weightlifting and a 3,700 calorie-a-day diet.

Skarsgård, 45, added about 20 lbs. of muscle for the movie with the help of trainer Magnus Lygdbäck, who helped him get in shape for 2016's The Legend of Tarzan. This time, they had to start from scratch with weightlifting.

"He's a skinny guy who likes to run. He doesn't go to the gym normally. Through running, he's got great lung capacity, but he doesn't really lift. And that means I can make a bigger change once I start working with someone like him," Lygdbäck told Entertainment Weekly.

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Lygdbäck put Skarsgård on a plan for six workouts a week, for one hour a day.

"I never train my clients for more than an hour because what doesn't happen in the first hour will not happen in the second hour at the gym. All you do after the first hour is just start to break down your body," he told Variety. "So one hour a day, six days a week. While filming, five days a week."

The workouts consisted of "a hybrid between plyometrics training and old-school bodybuilding philosophy," Lygdbäck said, using free weights and resistance bands. He also included bear-crawling and throwing exercises to get Skarsgård ready for scenes where he would scale walls and throw spears and axes.

"We knew Alex was going to be moving a lot, from running around to swinging axes, so we took a lot of attention to shoulders and hips," Lygdbäck said. "I did a lot of band warm-ups for his shoulders and hips, and then that was followed by pretty intense strength training. It was a four-day split, meaning you're working different muscles each day and then you've covered the whole body in four days. Then you start over."

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Along with the workouts, Lygdbäck advised Skarsgård on his nutrition, and had him eating around 3,700 calories a day to build as much muscle mass as possible. Skarsgård would eat five times a day, 2 to 3 hours apart to keep his "energy up and your metabolism burning," Lygdbäck said, and each meal would include a protein, a vegetable and a carbohydrate or fat.

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"[Alexander] loves to eat," Lygdbäck told EW. "I'm a big believer in not having any food restrictions. You might not want to eat pizza five days a week. But for me, 17 out of 20 meals should be on point, 3 out of 20 you should eat whatever you want. With that said, those 17 out of 20 meals that were clean and on point — he ate everything from steak to lamb to chicken, a lot of fish, eggs, quinoa, barley, farro, potatoes, even pasta and a ton of vegetables."

And the two friends would cook together for Skarsgård's "cheat meals" during filming.

"It was during [the] pandemic, so we're stuck in his house in Belfast," Lygdbäck said. "We would have a glass of wine, watch some soccer, and cook for hours. A casserole or steak, fish. Maybe with a nice little starter. It was more about the quantities — I would say that we're overeating and maybe adding a glass of wine."

Overall, Lygdbäck said, Skarsgård was a "dream client," and that's how he got those ripped results.

"He never complains. He's a hard worker. He will do whatever it takes," Lygdbäck said. "He's putting in that extra work, those extra reps. He will eat what you tell him to eat."

In Robert Eggers’ new epic “The Northman,” out this weekend, Alexander Skarsgård seeks revenge on a man who has killed both his parents and usurped the throne from his royal father. It’s a noble task, one that sees his character scaling mountains, rowing through tumultuous waters and getting into brutal, primal, combat. Tasked with such […]

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