Celebrity trainer Tracy Anderson recently told Redbook magazine that exercisers who only spin are likely to wind up with bulkier bodies.
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"'Spin may burn calories in the short term, but if that's all you're doing, it'll bulk up your thighs," she said in an interview published in the January issue of the magazine.
Anderson, who is actress Gwyneth Paltrow's business partner for a line of gyms and workout DVDs, added that the idea that spinning is a fast-track to slimming down is a common misconception.
"I have women who come into my office after spinning exclusively for six months, wondering why they can't fit into their jeans," she said.
Some people have interpreted Anderson's statements to mean that spinning makes one fatter. But Anderson clarified to ABC News that this is not what she meant.
"I've never said that spinning makes you gain weight," she explained. "What I've said is that spinning creates an imbalance in the muscular system. It bulks the thigh and butt muscles. You develop mass by working these same muscles over and over."
Even with the clarification, some fitness experts still aren't on board with Anderson's statements.
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"Unless that person has significant predisposition for laying down muscle mass, it's very unlikely spinning will result in massive quads or change in scale weight," Cedric Bryant, the chief science officer for the American Council on Exercise said.
While Anderson pointed to professional cyclists who have highly developed thigh muscles as proof that cycling blows up the muscles in the legs, Bryant said that idea is actually a tired myth.
"Top cyclists probably got into the sport because they have a natural predisposition to that body type in the first place. The average person doesn't have the genetic capability of developing that kind of mass even if they work out really hard," he said.
Other personal trainers in the same Redbook article did say that exercise could cause weight gain, probably because it stimulates appetite. Those people are really off base, according to James O. Hill, an exercise physiologist who is a professor of medicine at the University of Colorado School of Medicine in Denver.
"It's so wrong and there is so much data that clearly shows otherwise," he said.
Hill said that dozens of studies have found beyond a shadow of a doubt that dieters who add exercise to a weight-loss program lose more weight. Some research finds that a small percentage of exercisers, especially novices, do wind up eating a little more when they first begin working out, but they almost always wind up burning off more calories than they take in, he explained.
"This isn't a case where there's a lack of science to support a theory," Hill said. "This is one time where we clearly know the answer."
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Bryant said that spinning burns a respectable 500 calories or so in 45 minutes. As long as the spinning enthusiast mixes it up with some weight training and other cardio workouts to prevent burnout and injuries caused by overusing muscles, it's a great way to get in shape and maintain a healthy body weight.
He said he also believes it's a little irresponsible for anyone to suggest to someone who enjoys spinning that they are wasting their time.
"It's difficult enough to get people to exercise to begin with," he said. "It makes no sense to discourage someone against doing a workout they love."