Those roly-poly sneakers you are seeing everywhere -- with rounded soles like a rocking chair -- are marketed by manufacturers like Skechers, Reebok, Avia and New Balance with claims that they promote healthy weight loss, improve posture, fight cellulite, reduce knee joint stress and improve the shape of wearers' thighs and buttocks. The customer base for the shoes is 90 percent women, and they sell for $100 to $250, They also represent the fastest-growing segment of the athletic footwear industry. Skechers even hired Pro Football Hall of Fame quarterback Joe Montana to tout the shoes in an effort to entice more men to buy them. But doctors are starting to warn that the shoes destabilize how people walk, and wearing them can strain people's Achilles tendons. The shoes can also be a special threat to older people who have more difficulty keeping their balance. Their built-in instability makes a wearer work harder to maintain balance, giving more of a workout, according to companies that make them, and while there are some elements of truth to the ads, doctors question their safety and want more independent studies of the shoes instead of industry-financed research. Meanwhile, David Davidson, national president of the American Academy of Podiatric Sports Medicine, says "Nothing about these shoes has any redeeming value to me," and he says he's suspicious of any shoes that come with an instructional booklet and DVD.