NEW YORK (CBSNewYork) — Spin classes can be a great way to burn calories and stay in shape but if you over do it, it can be dangerous.

As CBS2’s Cindy Hsu reported, Roxanne Griffith loves to exercise, but spinning took a nasty turn.

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“I had no idea what it was. I just knew I had this terrible pain in my knee,” Griffith said. “I would be in agony.”

Turns out, that agony came from a meniscus tear in her knee from too much spinning, and she ended up having knee surgery.

“I was shocked,” she said.

Dr. Nadya Swedan said spinning is a safe exercise where you can burn more than 700 calories an hour in a very intense class. The danger is when you go overboard, spinning five to seven days a week and taking back-to-back classes.

“Unfortunately people can get injuries from their head down to their toes because you use your whole body in spinning,” said Dr. Swedan. “A lot of neck pain from repetitive spinning, that’s the only exercise they do.”

Dr. Swedan said mixing it up is the best way to stay fit.

“One day spinning, one day elliptical, one day a little jog,” she said.

Susan Rappaport struggled with obesity, but turned things around with fitness — even running several marathons.

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Now, she’s opened NuYu Revolution and created Posture Cycle. Rappaport said execution is more important than intensity.

“People focus on the intensity. So they’re very much in how hard they’re pedaling, how much tension they have, and this affects the body,” she said.

Rappaport said riders need to stop hunching over and straining their bodies.

“Posture cycle is all about the posture, no weight on the hands, shoulders back,” she said.

Rappaport said her riders use a fist to measure the correct position for the chin to protect the neck.

Lucy Fielding, 78, said spinning helps keep her healthy.

“The older you get, the better it gets,” Fielding said. “I’m dead serious!”

Now that’s inspiration to stay active.

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Fitness experts say you really need to listen to your body when you exercise. Don’t push yourself to the point of feeling sick, and that less is often more.