Joey King shares Nicole Kidman's brutal butt workout

Joey King has explained how she failed to keep up with Nicole Kidman when they worked out together.

The 24-year-old American actress co-stars with 57-year-old Australian icon Nicole in the new Netflix movie, A Family Affair.

And while the pair bonded behind-the-scenes of the romantic comedy, Joey says she was humbled when she tried to exercise with the actress who is more than twice her age.

During an appearance on The Tonight Show with Jimmy Fallon on Monday, Joey revealed, "Nicole taught me like the most epic, awful butt workout I've ever learned in my life.

"I was like, 'I'm a youngster. I can hang...I'm agile.' I thought I could hang, but I couldn't hang. It was all these donkey kicks, and the rainbows, and you know, the fire hydrants. But you have to keep your leg in the air for like 12 years. It's so hard."

Nicole has previously shared her love for exercise - saying her secret to avoid getting bored of workouts is to mix up her routines.

She told the Los Angeles Times previously, "If you think you are going to wake up and run five miles every morning for the rest of your life, you'll get bored and hit the snooze button after a week.

"So I try and mix it up with sports, running, yoga and even just going for a walk with my husband and the kids. Living in Nashville makes this easier, as it's so nice just being outside. I also love spinning. I try to get everyone to spin."