PORTLAND, Ore. - Reebok will need to tone down advertising for its shoes that claim to reshape your backside.

The athletic shoe and clothing company will pay $25 million in customer refunds to settle charges by the Federal Trade Commission that it falsely advertised that its "toning" shoes could measurably strengthen the muscles in the legs, thighs and buttocks. As part of the settlement, Reebok also is barred from making some of these claims without scientific evidence.

"Settling does not mean we agree with the FTC's allegations," Dan Sarro, a Reebok spokesman, said in a statement Wednesday. "We do not. We have received overwhelmingly enthusiastic feedback from thousands of EasyTone customers."

It's the latest controversy surrounding so-called toning shoes, which are designed with a rounded or otherwise unstable sole. Shoemakers say the shoes force wearers to use more muscle to maintain balance and consumers clamored for them, turning toning shoes into a $1.1 billion market in just a few years. Companies such as Reebok, New Balance and Skechers have faced lawsuits over their advertising claims. But the FTC settlement, announced Wednesday, is the first time the government has stepped in.

Reebok International Ltd. makes a range of toning products, including its RunTone running shoes, EasyTone walking shoes and flip flops and some clothing. The company, which is owned by Adidas AG, said that its toning shoes were one of its most popular product launches ever when they debuted in 2009. The company marketed them heavily with ads featuring women in short shorts and with shapely bottoms; one ad even said the shoes would "make your boobs jealous".

The FTC took issue with Reebok's ads that claimed its EasyTone footwear had been proven to lead to 28 percent more strength and tone in the buttock muscles and 11 percent more strength and tone in hamstring and calf muscles than regular walking shoes. The FTC said it could not disclose if it was pursuing similar actions against other shoe makers.

"We think this is a real victory for consumers," said Dana Barragate, an FTC attorney involved in the case. "We hope it sends a message to businesses that if they are going to make claims they must be justified."

Shoe makers, including Reebok, have funded studies and say they have anecdotal evidence that proves they are effective. Several experts have questioned their validity and the American Council on Exercise, a nonprofit fitness organization, conducted a study that found toning shoes failed to live up to the claims of shoe makers. However, the council said the shoes could be beneficial to one's health if they motivate people to get moving.

Christopher Svezia, with the Susquehanna Financial Group, said many shoemakers have changed their advertising approach as criticism has mounted. "The emphasis has moved to fitness instead of making these kinds of claims and promises," he said. "The question is who is next and how much is it going to cost them."

The industry has faced other issues. There have been some injuries reported by wearers who have found themselves with shin splints, twisted ankles and sore muscles from the new gear and motions. Shoe makers suggest new wearers ease into wearing them.

Toning shoes were once the fastest-growing segment in the footwear industry, but recently lost some ground. SportsOne Source Group said that the $1.1 billion market of 2010 is expected to fall about 40 percent to $650 million in 2011 after Skechers flooded the market with products, forcing prices down. However, SportsOne Source said the number of shoes sold is only expected to fall 5 percent, suggesting there is still fairly strong demand.

Rebecca Sayre of Seattle, who bought a pair of Skechers more than a year ago, said they made her legs stronger and posture better. But, she says: "They've lost their luster."