Joey King, 24, revealed she couldn't keep up with Kidman's, 57, intense workout routine in an interview on The Tonight Show Starring Jimmy Fallon.
"She taught me the most epic, awful butt workout I've ever learned in my life. It was so intense," King said.
"I was like, 'I'm a youngster. I can hang. I'm young, and I'm agile' — I thought I could hang, but I couldn't hang."
King went on to say she still does the workout, which consists of donkey kicks, rainbows, and fire hydrants, because it's "the most effective:"
"You have to keep your leg in the air for, like, twelve years, it's so hard," King told Fallon, adding that she often will text Kidman photos of herself doing the exercises, joking that she should "call an ambulance."
King stars as Kidman's daughter in the new Netflix film A Family Affair, which also stars Zac Efron.
"They're both like so hot," King said of Kidman and Efron. "They're like the fittest people I've ever met, so, I was inspired by them. So I became a bit of a, you know, tubular gym rat on the set."