WASHINGTON, Sept 28 (Reuters) - Reebok International Ltd has agreed to pay $25 million to settle charges that it made unsupported claims that its "toning shoes" provide extra muscle strength, the U.S. Federal Trade Commission said on Wednesday.

Image from reebok.com

The money will go toward consumer refunds.

Reebok advertisements said the shoes strengthened hamstrings and calves by up to 11 percent more than regular sneakers, and toned the buttocks by up to 28 percent more, the FTC said.

"To its credit, Reebok pulled these ads sometime in the middle of our investigation," David Vladeck, head of the FTC's Consumer Protection Bureau.

Toning shoes are designed to be slightly unstable. Makers of such shoes often say the instability requires the wearer to work harder, thus strengthening muscles.

"We did get consumer complaints. We watch TV. We read the newspapers," said Vladeck. "There is no such thing as a no-work, no-sweat way to a fit and healthy body."

Adidas (ADSGn.DE), which owns Reebok, said in a statement that it disagreed with the FTC and stood behind the shoes.

"The (FTC) allegations suggested that the testing we conducted did not substantiate certain claims used in the advertising of our EasyTone line of products," Adidas said. "In order to avoid a protracted legal battle, Reebok has chosen to settle with the FTC. Settling does not mean we agreed with the FTC's allegations; we do not."

The company added, "We stand behind our EasyTone technology -- the first shoe in the toning category that was inspired by balance-ball training."

A variety of companies advertise toning shoes, including New Balance, Skechers (SKX.N), Ryka and Avia.

Skechers acknowledged in an August filing with the Securities and Exchange Commission that the FTC was looking at its advertisements for its Shape-ups and other toning shoes.

The FTC said Reebok began making the claims about its EasyTone and RunTone shoes in early 2009, and provided statistics about the purported benefits of the shoes.

The refunds to customers will be made available either directly from the FTC or through a court-approved class-action lawsuit, the agency said. (Reporting by Diane Bartz in Washington and Nivedita Bhattacharjee in Bangalore; editing by Gerald E. McCormick and John Wallace)