Sports goods maker Reebok International is to pay $25m (£16m) to settle charges that it made unsupported claims about its Easy Tone and Run Tone shoes.
Reebok, a unit of Adidas, said these toning shoes would "strengthen and tone key leg and buttock (gluteus maximus) muscles more than regular shoes".
The US Federal Trade Commission ruled these advertising claims were false.
A spokesman for Reebok, based in Massachusetts in the US, was not immediately available for comment.
The FTC said Reebok began making the claims in early 2009 and provided statistics about the alleged benefits.
The $25m penalty will go towards consumer refunds.
"The FTC wants national advertisers to understand that they must exercise some responsibility and ensure that their claims for fitness gear are supported by sound science," said David Vladeck, director of the FTC's bureau of consumer protection.
The commission said in one advert Reebok claimed that by walking in its Easy Tone shoes users were able to strengthen hamstrings and calves by up to 11%, and tone the buttocks up to 28% more than normal trainers.
It comes three months after a Reebok advert in the UK, which featured Formula One driver Lewis Hamilton, was banned.
The Advertising Standards Authority (ASA) banned the leaflet which said Reebok's ZigTech Apparel helped blood vessels to relax, boosting oxygen levels by up to 7%.
The ASA said the claims could not be proved and also criticised the advert for implying the trainers Hamilton wore in it featured the new technology.
Reebok said it disagreed with the ASA ruling but accepted it.