They came with the promise of a firm tush and taut thighs, but what toning shoes have delivered are claims of injuries from consumers, headaches for their makers and a bounty of legal work for plaintiffs firms.

At least 90 personal injury lawsuits are pending in federal and state court against Skechers USA Inc., maker of Shape Ups, the shoe with a thick, curved sole touted as helping people who wear them during everyday activities lose weight and get fit. The plaintiffs claim that, rather than providing an easy way to slim down, the round-bottomed shoes are dangerously unstable, resulting in broken ankles, busted knees, even head injuries. Skechers has the biggest share of the toning-shoe market, which dropped from $1.1 billion in 2010 to about $550 million in sales last year, according to SportsOneSource, a research firm in Charlotte, N.C. Skechers has about 49 percent of the market while Reebok International Ltd. and a handful of other manufacturers control the rest, said Matt Powell, a SportsOneSource analyst.