Women who have invested hours at the gym trying to shrink their bottom line are now booting up their "assets" at the plastic surgeon's office.

They're sub-muscle silicone implants for the backside, and they are quickly becoming the breast implants of the new millennium.

"There is much more of a demand for this type of surgery," said New York City surgeon Dr. Bruce Nadler. "What Pamela Anderson did for the chest, Jennifer Lopez has done for this part of the anatomy."

At $4,250 per bun, an enhanced rear end isn't cheap. But Nadler said the procedure isn't just for the rich and famous.

"You can finance the surgery in the same manner you would a car," he said. "And this is an investment in your self-esteem — a car you have to trade in after a few years."

One of Nadler's patients, a 27-year-old resident of Bristol, Conn., said having the surgery completely changed her self-image.

"In that area, I had nothing at all, no shape, no cushioning" said the 5-foot, 100-pound former gymnast, who has also had a nose job. "I just went on vacation and I even wore a thong on the beach. I never would have done that before."

But padding one's tush can literally be a major pain in the butt. Recovery time can last more than six weeks. In the gymnast's case, it took two tries to get the surgery right, along with months of "nightmarish" soreness and weeks of missed work.

That's exactly why Georgia plastic surgeon Dr. Richard Greco does not perform the operation. "I find that the difficulties involved in getting a good result and the number of possible complications are greater than the result," he said.

Greco listed infection, displacement, bleeding, discomfort and other sensory problems as possible side effects of buttock implants.

But he has an alternative. The doctor instead recommends and often performs fat injection surgery, which he says is better for the area that "gets so much wear and tear."

Yet another problem is the feel of the implants. While the new, softer inserts are better than their previous incarnation, Nadler's patient says her backside is still "really hard." In other words, your partner will figure out your buns are not naturally made of steel, even if there is no visible scar.

Carolina Miranda, contributing editor at Latina magazine, said she "had to laugh" about the J-Lo-inspired look. "It's a weird compliment to how far Latinas have come, but we'll take it," she said.

Nadler said chubbier cheeks used to be coveted mainly among Hispanics and Asians who had "very small endowment there." But now women of all types want to fill out a pair of jeans.

Not everyone, however, is behind the big new trend. New York City teacher Patricia Jones had a hard time even believing it was for real.

"It's ridiculous. Everyone's running out exercising, trying to be skinny like a boy. And now they're paying for big butts," she said. "I hope the next trend will be to be bigger all over. Then we won't have to do anything anymore. We could just eat a lot."