10 Things Your Personal Trainer Won't Tell You

And yet not all so-called specialists are properly trained — on the fast-and-loose end of the spectrum, you'll find certification requirements as minimal as a $500 fee and passing an online exam. That worries John Buse, president of medicine and science at the American Diabetes Association, because when exercise isn't done properly, any vision problems and nerve damage in the feet that some diabetics develop could worsen, he says, in extreme cases to the point of blindness or amputation.

Given that 37% of health club members are beginners, personal trainers are largely catering to the unfit, according to IDEA Health & Fitness Association, a San Diego-based organization for fitness professionals. They're reaching out to seniors as well, since clients 55 years and older constitute one of the fastest-growing segments of gym members, says IDEA's executive director, Kathie Davis. Nonetheless, many trainers are guiding clients with a less-than-gentle hand. "The majority of people that come into the club haven't worked out since their high school gym class," Thomas says. "Then we're told to work them hard. It's dangerous." If you feel your trainer is being too tough, speak up. Remember, you're the boss.

This niche is growing because our kids are: Roughly 15% of American children are overweight, ranging from a high of 22.8% in Washington, D.C., to a low of 8.5% in Utah, according to nonprofit Trust for America's Health. But not all health clubs have trainers who work well with kids — or even know how to work them out safely, says Davis. Even a good trainer with the wrong attitude can turn impressionable kids off to working out.

Bottom line: Be selective. For starters, ask for a trainer with a background in teaching, coaching or child development, Davis says. And if your kid is involved in a particular sport, requesting a trainer with a similar background can help develop specific muscles and prevent injuries.

Though health clubs don't typically dangle the group option in front of you, most personal trainers will work something out if you ask. After all, it's a win-win situation. For a group of three, for example, the average fee of $60 per hour is reduced by half for each client, while the trainer brings in about 50% more than he typically makes in an hour. And it could mean a better workout: "There's a lot to benefit from group camaraderie, as long as you don't need a trainer counting every rep you do," says Richard Cotton, national director of certification for the American College of Sports Medicine.