Exercise fads infiltrate gyms faster than you can say "cardio strip," but these new routines can be dangerous for people hopping on the fitness bandwagon.

"Jumping on something too fast can be a mistake," said David Kirsch, owner of the New York gym Madison Square Club. "Tae Bo was once the latest and greatest craze, but where is it now?"

Yoga is one trend that seems far from extinction. Approximately 15 million people practiced yoga in the United States, a figure that has almost doubled in the past five years, according to Yoga Journal.

But the rush in popularity has led to some cases of novice instructors teaching the masses, according to Leslie Kaminoff, a yoga therapist who has treated many injured yoga enthusiasts.

And as people push themselves to keep up with advanced classes, they get hurt. "I've had clients who've been injured by domino effect," he said, which happens when students fall into each other while doing inverted positions like headstands.

He compares the yoga trend to the aerobics craze in the 1980s. "If you look at Jane Fonda's first tape, it's like, 'How to injure yourself to music,'" he said.

But chronic injuries are more pervasive than acute ones, he said. One such chronic injury is what Kaminoff calls "yoga butt."

"It's a pain that starts in the butt and goes down back of the leg," he said. "It develops over time because the sequencing used in some classes involves long extended periods of time sitting on the butt ... then twisting and bending into positions."

But chronic pain can come in many forms. Meyung Kim, 31, who began practicing yoga four months ago has already gotten hurt. "I pulled the muscle underneath my shoulder blade, performing a shoulder stand during a class," she said. "It started feeling better and I went back and now it hurts again."

Kim said she works out regularly, but started taking yoga at the gym to try something new. These days, people looking for the latest fad will likely come across kettlebells, a trend just hitting the U.S.

Originally used as a bodybuilding tool by Russian soldiers and weight lifters, kettlebells are used like weights to build strength and provide a cardiovascular workout, according to David Ganulin, head of the kettlebell program at Equinox gym.

"It's extremely efficient. It trains the body as a unit, instead of a collection of body parts," he explained.

The weights, shaped like a bowling ball with a handle, have traditionally been used by men and can weigh up to 88 pounds. But companies like Dragon Door have made lighter bells for ladies starting at 9 pounds. And a new book, From Russia With Tough Love by Pavel Tsatsouline, a former instructor for the Soviet Special Forces, creates a kettlebell routine specifically for women.

But experts say even with at a lighter weight, kettlebells pose a risk of injury.

"It seems too cumbersome," said Kirsch. "I think it's incredibly dangerous to use them on your own. People are going to get hurt if they do this."

And he added that just because bodybuilders have used them, doesn't make kettlebells appropriate for average gym-goers.

"I'm sure strength and power people have been using kettlebells for years but that doesn't necessarily mean they will work in a general training program."

But Equinox, which is launching a new kettlebell program this month, says safety is emphasized.

"There will be one instructor and no more than three people per session," said Jason Frye, Equinox's public relations manager. "It's a weight-lifting class and requires more individualized attention."

Kirsch said ultimately, he'd rather stick with weights and dumbbells than try a dubious new trend. "I know what works and I stick with what works," he said.

But for those who want try the latest fads, the best way to approach any new workout is incrementally, said Kirsch.

"Whether kettlebells, yoga or anything relatively new, you want to gradually step into it," he said. If not, your injury could linger longer than the craze.