Indoor-cycle much? Unless you've been locked in a room with only step aerobics tapes for company, someone has probably tried to convert you to the Cult of Spin. Maybe you're already part of the ever-increasing army of indoor-cycling addicts who will do anything for a fix, not scheduling meetings when the SoulCycle booking windows open on Monday at noon and hitting five to 10 classes a week. (Yes, some of these may even be "doubles," as back-to-back classes are known.) There's no doubt that not-so-easy cycling torches calories—about 400 to 600 per 45 minutes—so why are some Spinsters (reluctantly) cutting down or, in some cases, swearing off it entirely?

First, depending on the body you start with, indoor cycling may not get you the body you want. Says celebrity trainer David Kirsch, "If you have a predisposition to bulking in your lower half, Spinning can make your butt and quads bigger." Adds Rebecca Battista, an associate professor of exercise science at Appalachian State University, "Those are the muscles you're using. Some cyclists get really big thighs." As for the recent trend of aqua cycling (bikes in the pool), it's too soon to tell whether the impact on thigh size can be minimized, says James Pivarnik, a professor of kinesiology at Michigan State University.

Kirsch, who notes that he is "not anti-Spinning," rations clients' classes—he lets, say, an apple-shaped woman with skinny legs go once a week—but forbids fashion models to ride at all. Michael Ciardulli Jr., the bodyworker of choice for New York's A-listers, warns clients against it: "To be lean, you need to lengthen the musculature; cycling can shorten it." Julie*, a 30-year-old publicist, gave up her first love, running, when she fell head over heels for indoor cycling. She was less than ecstatic, however, when after a few months, "my butt felt and looked padded and my legs felt heavy." She gave up all exercise for a month to let the muscles atrophy, then switched to yoga. "My lean, shapely runner's legs are back," she says. Ditto Erica*, a 49-year-old indoor-cycling instructor who gave up teaching eight classes a week after she noticed that her jeans were getting "really tight, uncomfortable in the butt and thighs." She swapped the bike for barre classes and running and says she dropped 10 pounds. Of course, not everyone agrees that the sport amplifies your bottom half. Jennifer Sage, founder of the Indoor Cycling Association, wasn't surprised to hear of results like Julie's and Erica's but is adamant that "very few people will get larger legs." And, she adds, if they do, "they could probably stand to lose a little bit of the body fat on top." Ouch. So who gets big and who doesn't? Pivarnik is noncommittal. "Not everybody responds to the same exercise stimuli the same way," he says.

No discussion of gaining weight and bulking up would be complete without considering diet. Another reason Spinsters are logging less time on the bike: Cycling makes most people extremely hungry. That's why Hollywood trainer Harley Pasternak instructs followers of his Body Reset Diet to "give up your Spin class." Says Pasternak, "I call it the carte blanche effect: 'I did a kick-ass class today. I deserve dessert.'" Fitness instructor Mandy Ingber, who works with Jennifer Aniston, has experienced the hunger firsthand. Though she used to teach upward of 10 classes a week, she has cut down drastically since opting to expand her yoga practice. Now Ingber's on a bike three or four times a week and says she doesn't need as much food. Also, she says, "my body is more balanced."

Finally, every cycling class is a killer workout. What could possibly be wrong with that? "Your body needs an easy day, and that's hard when you're in a class where everyone is pushing," Battista explains. "If you just build, build, build intensity, your ability to achieve results plateaus." If you want to keep clipping in, experts recommend maxing out at three times a week and alternating with weight training. A 2011 study showed that about 15 minutes of strength training can boost your metabolism for three days. "Compare that to intense cycling, where you get maybe half a day of afterburn," she says. Barbara Crean, a trainer at New York's Clay gym, suggests looking in the mirror. "It comes down to this: Do you like the results?" says Crean, who cycles at Flywheel. "It may be time to change something." And she doesn't mean the resistance.

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