Alexander Skarsgård gained 20 pounds of muscle to sculpt a thick viking look for his role in 'The Northman,' his trainer said. Here's how he did it.

• Alexander Skarsgård gained 20 pounds for the upcoming viking thriller "The Northman."
• He worked out four times a week, lifting both light and heavy weights, his trainer told Insider.
• Skarsgård ate lots of protein in a slight calorie surplus, Magnus Lygdbäck said.

Alexander Skarsgård put on 20 pounds of muscle for his role in the viking thriller "The Northman," set to hit theaters in April.

The Swedish actor's trainer and nutritionist, Magnus Lygdbäck, spoke with Insider about how they built his physique for the role.

Skarsgård trained with both heavy and light weights and aimed to eat in a slight calorie surplus with no foods off-limits, Lygdbäck said.

It wasn't the first time Lygdbäck had worked with Skarsgård. He'd previously trained the actor for his role as Tarzan. The goal was slightly different for "The Northman," though.

"We wanted Alex to have a little thicker look, a little more viking," Lygdbäck said. "We needed him to look like a bear but move like a wolf."

Lygdbäck trained Skarsgård for about three months before shooting started in March 2020, but when the coronavirus hit, the movie was put on hold.

Skarsgård then had to maintain his physique as best he could without a gym for a few months before Lygdbäck was able to train him again from June to the end of August. But this ultimately meant Skarsgård was in even better shape when they finally started shooting, Lygdbäck said.

Skarsgård trained four times a week

The main goal of Skarsgård's training was muscle gain, without adding fat to his already-lean physique.

"We wanted him to have low body fat, but we didn't want him to be as shredded as Tarzan," Lygdbäck said. "It was more about building that power and looking powerful."

Lygdbäck said he also took into account Skarsgård's age. Keeping him injury-free at 45 was important, so they did lots of mobility work.

Skarsgård trained four times a week, with a potential fifth session of active recovery or mobility, Lygdbäck said.

Each training session included some cardio through high-intensity interval training and then strength work, Lygdbäck said.

For hypertrophy, or muscle gain, trainers usually recommend lifting weights in sets of eight to 12 reps. But Skarsgård did both lower and higher reps to keep himself injury-free. If Skarsgård arrived at the gym exhausted, Lygdbäck knew he'd be at risk of hurting himself, so they did lower weights or machine work instead of heavy compound lifts.

Skarsgård's nutrition was key

Lygdbäck said getting Skarsgård's nutrition on point was more important than training.

The aim was to eat at "the sweet spot" of slightly above maintenance calories. Three weeks before shooting, Skarsgård ate in a slight calorie deficit, Lygdbäck said.

Skarsgård tracked his macros (macronutrients, or protein, carbs, and fat), but the quantities varied slightly over the training period. Lygdbäck ensured the actor always kept his protein intake high to aid recovery and muscle building.

Lygdbäck encourages people to aim for 1 gram of protein per pound of body weight per day, constituting about 30% of total food intake.

To hit his protein target, Skarsgård ate a lot of eggs (they were his go-to breakfast) as well as fish, chicken, and beef.

No food was off-limits, which is part of Lygdbäck's "17/20" nutrition philosophy. 

"I gave him a slack on the weekends," Lygdbäck said. He and Skarsgård regularly cooked together and enjoyed a glass of wine.

To look like a viking, work hard and be patient

For people who want to achieve the viking look, Lygdbäck said it takes a lot of hard work. They shouldn't expect overnight results. 

"It's a marathon, not a sprint," he said. "There are no shortcuts. That means hard work when you're in the gym — you've got to get on top of training, nutrition, and recovery." 

But Lygdbäck said people shouldn't be too hard on themselves as they work toward their goals.

"You might be working too hard, and you do need a day off, or you might be fooling yourself and you're being lazy," he said. "You've got to find that sweet spot."