The TV and magazine campaign received two complaints that the ads were misleading and unsubstantiated.
The TV ad shows women walking, dancing, jumping, standing and spinning around while wearing the trainers, as the camera focuses on their bums and legs.
The voiceover said: "Reebok EasyTone. Helps tone legs and bum more than regular trainers. Reebok EasyTone with balanced ball inspired technology. Better legs and better bum with every step."
The accompanying magazine ad shows an outline of a woman underneath the headline "Reetone with every step. Get up to 28% more of a work out for your bum. And up to 11% more for your hamstrings and calves".
Reebok defended the campaign with an independent study it had commissioned, which compared the increase in muscle activation in certain parts of the body when wearing the trainers and when walking barefoot.
Although Clearcast cleared the ad, saying Reebok's claims were substantiated by the study, the ASA dismissed the evidence and upheld the complaints.
The ASA said the sample size of the study was "very small" and was "not adequate to support the absolute claims made in the ads that consumers would achieve an improvement in muscle tone".
The ASA ruled the ads were misleading, as Reebok did not provide "robust, scientific evidence" to support the claims.
A Reebok spokeswoman supplied the following statement: "EasyTone shoes use balance ball-inspired technology. Balance balls are used in gyms around the world and the benefits of this type of training are well documented. Despite these two complaints to the ASA, thousands of consumers have told us they love our shoes and that they work. For us, that's the most powerful evidence."