Katheryn Winnick plays Lagertha on Vikings, which premieres its fourth season Thursday

Credit: Jonathan Hession/History Channel

It’s no surprise when Katheryn Winnick says she’s “not a girly girl.”

“I’m more of a tomboy,” the actress tells PEOPLE before the premiere of the Vikings fourth season on Thursday night.

“I absolutely love punching and kicking, and kicking the crap out of somebody,” she says, referring to her many fight scenes as Lagertha in the Michael Hirst drama, and to her “first love” – martial arts.

Winnick, 38, started taekwondo at the age of seven, got her first black belt by 13, and started her own school by 16. “By the time I was 21, I had opened up three different schools in Toronto. I started teaching actors martial arts on movie sets, and my first passion, my first love was martial arts.” The actress holds a third degree black belt in taekwondo and a second degree black belt in karate.

“Lagertha’s role is very similar to me,” she says of her regal, battle-ready character. “In the sense of determination and working hard and being able to defend yourself and having the knowledge you need to be able to get out of any situation. With that comes an inner strength – how you deal with conflict. Her being so strong and being a force to be reckoned with comes from growing up as a shield maiden, and for me, growing up as a martial artist.”

Filming the show in Ireland gives Winnick “access to incredible landscape and great hikes and mountains and horses and believe it or not even swimming, although right now it’s a little cold.”

Winnick says she also does weights and kickboxing to stay fit, but adds it’s more important to focus on getting strong than on appearance. “Strong is beautiful. Strong is sexy. I truly believe that,” she tells PEOPLE. “If you walk into a room being confident, that’s sexy. I just think that every girl should feel strong.”

“I’m in an industry where you’re under a microscope,” she says. “But I try not to worry about that. I try to focus on myself and working on an inner strength and working on peace and confidence and trying on different characters.”

“What infuriates me a lot of the time is when we idolize some actresses and celebrities who starve themselves or who are so skinny, or who are so image-conscious, but we forget that women should be strong too and that that strength should be celebrated,” says Winnick, who works with the organizations Women in Film and Girl Up to promote empowerment for women and girls.

“You can be empowered and be a force to be reckoned with, and that is beautiful.”

Vikings season four premieres Thursday at 10 p.m. ET on History.