Marisa Miller is quite fond of her derriere, but she admits that wasn't always the case.

In the February issue of Shape, the Sports Illustrated swimsuit model, 32, reveals that she was given the nickname "bubble butt" in high school.

"We refused to do squats at volleyball practice because we didn't want them to get any bigger," she tells the mag, which she's graced eight times. And now? "I love my butt and want to make sure it stays where it is," she says. "I was just talking to that same friend and we were laughing because now all we want to do is lots of squats to keep our rear ends curvy and feeling strong."

That's not all the married star — she wed music producer Griffin Guess in 2006 — does to keep her fab figure. She also steps into the boxing ring for 90 minutes up to four times a week.

"I'm one of those people who likes to sweat," she says. "I've tried Pilates and yoga, but they're a little too meditative for me. I need to really go for it, and do it hard!"

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Her routine? Fifteen to 20 minutes of jump rope, 3-minute rounds on the punching bag and five sets of 30 to 40 squats.

"I go to a down-and-dirty boxing gym," says Miller, who unwinds by riding her Harley-Davidson Nightster. "I don't want to worry about how I look or whether I’m wearing the perfect outfit. For me, it’s about focusing for an hour and a half on my workout."

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