Exercise Ball

Bernard Yang Kim never wanted to be a bodybuilder. The 31-year-old currency trader simply wanted to look chiseled—like an underwear model, he jokes—which is why he found it odd to be staring up at a 315-pound barbell. He had never benched so much weight in his life; few men ever do. But his usual trainer was out, and his gym had set him up with a substitute—one who, as it turns out, was not only overzealous but also a terrible spotter. "The bar crashed onto my chest, tearing my pectoral muscle," said Kim, who ended up in the ER. "It was excruciating."While extreme, Kim's experience is not uncommon. There are roughly 230,000 personal trainers in the United States, a number that has jumped 44 percent in the last decade. Indeed, personal training is one of the few professions to not only blossom during a recession but also grow afterward as people turn to it for a second job and even a second career. And it's easier than ever to get certified: You can go online, take a course, and start t...

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