Vikander takes over from Angelina Jolie in the new film, which is released later this month.
It’s action-heavy film that required the Oscar winner to be on top of her game with her fitness.
Speaking on ITV’s Lorraine today, she admitted that she was nowhere near ready for all the gruelling stunts when she first took the role.
“I could not lift my own weight when I started this film – I [needed] to get this kind of physique,” she said.
“We talked about what I wanted to achieve. I started to work out three or four months before we started the shoot.
“I trained with some incredible women. They introduced me to boxing, MMA [Mixed Martial Arts], I did rock climbing and archery, swimming, cycling…
“It was really a wonderful experience. I was a dancer when I was a younger, I’ve always been intrigued by the physicality of making these kinds of films.”
But of maintaining her “8-pack”, she laughed: “It all goes away sadly, very quickly.”
The new Tomb Raider also stars Dominic West, Daniel Wu and Kirstin Scott-Thomas, and has its premiere in London tonight.
Jolie played Croft in two movies, back in 2001 and 2003.
Tomb Raider is out on March 16.