Blake Lively's body "shifted and changed" after she was pregnant with her first child.

Blake Lively

Blake Lively

The 28-year-old beauty hired a personal trainer to get back into shape following the birth of 18-month-old daughter James Reynolds but admitted the exercise did more for than just help her shed the baby weight she'd gained.

She said: "I worked out with an amazing trainer Don Saladino. He kicked my butt. It was also eight months after having a baby so my body was already completely different than what it normally was. Not just aesthetically but also your muscles are different, your body, everything is just shifted and changed.

The former 'Gossip Girl' star was training for her forthcoming movie 'The Shallows' and though it was hard work and felt like she was preparing for a "marathon", she was glad to have a reason to get fit.

She told E! News: "It was really crazy. But it was also nice to have that motivation because I wouldn't have done that. I wouldn't have gotten in that kind of shape.

"It was like training for a marathon, but a marathon is one day. This was six weeks straight, six-day weeks, no lunch breaks, thirteen-hour days, four-foot waves at all times. By the time I was done with the movie...I had muscles that I had never had before."

Meanwhile Blake - who is married to 'Deadpool' star Ryan Reynolds and is expecting their second child - confessed she finds it "torturous" watching her husband's sex scenes.

She said: "It's kind of torture these days because I'm on a plane and every screen is my husband Ryan in a sex montage throughout the holidays with another woman because everyone wants to watch 'Deadpool' on the plane, everyone!

"But for like 14 hours having your husband having like mashed potatoes eaten out of his butthole - that's in the film - it's very, it's lovely. It's a cruel and unusual form of torture."