NEW YORK (AP) — Gym classes that promise a plump posterior are in high demand. A surgery that pumps fat into the buttocks is gaining popularity. And padded panties that give the appearance of a rounder rump are selling out.

The U.S. booty business is getting a big bump. Companies are cashing in on growing demand from women seeking the more curvaceous figures of their favorite stars, who flaunt their fuller rear ends.

Nicki Minaj, for instance, raps about her "big fat" butt in "Anaconda." Reality star Kim Kardashian posts photos of hers on Instagram. And in the music video for "Booty," Jennifer Lopez and Iggy Azalea, wearing leotards, spend four minutes rubbing their curvy bottoms together. At one point, they slap each other on the booty.

As a result of the pop culture moment the butt is having, sales for Booty Pop, which hawks $22 foam padded panties on its website, are up 47 percent in the last six months from the same period a year earlier. The company, which declined to give sales figures, has sold out of certain styles and colors this year, including its Pink Cotton Candy Boy Shorts.

Susan Bloomstone, Booty Pop's co-founder, says customers have asked for larger sizes. So, the Boston-based company plans to begin selling pads that are 25 percent larger this month. "People just want more booty," she says.

Feel Foxy, another maker of padded panties, says 2014 has been its best year since launching nearly a decade ago. Sales are up 40 percent from a year ago, but the company declined to give sales figures.

"The Nicki Minaj song gave women the idea to pay attention to their rear end," says Jessica Asmar, co-owner of the Houston company.

Deborah Santiago squeezed into a $40 Feel Foxy one-piece for her 30th birthday. The shapewear flattened Santiago's waist and boosted her back side. A flat butt can ruin an outfit, says the New York stay-at-home mother of two. Lopez is her butt idol, but she also covets the bottoms of reality TV stars on "The Real Housewives of Atlanta" and "Love & Hip Hop."

"I always wanted a big butt," Santiago says. "Something you could look twice at."

To be sure, the desire for big butts isn't new. Large booties long have been preferable in Latino and black communities, says Dr. Dionne Stephens, an associate psychology professor at Florida International University who has researched sexuality in popular culture. And this isn't the first time big butts have been in songs. (Think: "Baby Got Back" by Sir Mix-A-Lot in the 1990s.)

But recently, the desire for a bigger bottom became more mainstream, in large part due to pop culture influences. Mainstream celebrities like Lopez and Minaj accepting their ample assets on camera have given the butt cachet. "When people see things repeated on TV more and more, it becomes normalized," Stephens says.

French sociologist Jean-Claude Kaufmann says this is true overseas, too: "In Europe, and in France especially, there's a trend to show off the buttocks in place of breasts. This has to do with Latin American influences, but also the rise of Beyonce and stars like Rihanna," says Kaufmann, author of "Women's Bodies, Men's Gaze. Sociology of Naked Breasts."

Kaufmann also suggests more economic reasons at play: "In uncertain times, people look for security," he says. "Men are attracted to women's hips and the buttocks for security and reassurance. Women respond to this. It's deeply psychological."

Whatever the reason, the widespread interest in larger hind parts seems to have started when Kardashian's reality TV show, "Keeping up with the Kardashians," began airing seven years ago.

In a 2011 episode, she had an X-ray to prove she didn't have butt implants. Kardashian still frequently posts shots of her backside to her 21 million Instagram followers.

But the desire for big buns has intensified. This summer, the music video for "Anaconda" that showed Minaj in a pink thong was viewed 19.6 million times within 24 hours of its release — a record for music video site Vevo. It has racked up nearly 300 million views. The song has been on the top of the Billboard charts, too, right behind another anthem for curvy women, Meghan Trainor's "All About That Bass."

"I'm bringing booty back," Trainor sings.

Some businesses that specialize in butts say pop culture has had a direct impact on their bottom line.

A Brazilian butt lift, in which fat is sucked from a patient's stomach, love handles or back and put into their buttocks and hips, is increasingly popular in the U.S. This type of surgery, along with buttock implants, was the fastest-growing plastic surgery last year, with more than 11,000 procedures, up 58 percent from 2012, according to the American Society for Aesthetic Plastic Surgery.

Dr. Matthew Schulman, who performs the procedure in New York, says this year has been busier than last. Schulman, who charges $10,000 to $13,000 for the three-hour surgery, does six to eight Brazilian butt lifts weekly, up about 25 percent from a year ago.

He says when he asks patients which celebrity butt they want, the top names are Kardashian, Minaj and Lopez. Recently, more women have asked for a butt like Kardashian's sister, Khloe, who also stars in the reality show.

The downside of new interest is that women desperate for cheap options have risked their lives, going to phony doctors that inject silicone, and even bathroom caulk, into their buttocks. Deaths have been reported in Miami, New York, Las Vegas and Jackson, Mississippi.

Another problem is doctors performing butt-enhancement surgeries that don't have experience. Schulman says about 20 percent of his patients come to him so he can fix lumps, bumps and uneven butt cheeks done by unskilled doctors.

Not everyone is trying surgery, though. Those looking for more natural ways to enhance their derriere are attending workout classes and watching workout videos that target the butt.

DailyBurn, which streams workout videos, says views for its "Butt, Hips and Thighs" video doubled in January and have remained popular. The video is so popular that DailyBurn is adding another butt workout clip in December.

At a gym in Boston, there's a waitlist for a $30 class that fits in 120 squats in 45 minutes. The class, Booty by Brabants, was started by Kelly Brabants a year ago. Brabants starts most classes, held at The Club by George Foreman III gym, with Lopez's "Booty" song. By the end of the year, she plans to expand her brand by selling $65 workout leggings that help perk up the butt.

"It's not about being stick-thin anymore," says Brabants. "Every girl now wants a booty."

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