Spinning might look about the same as outdoor cycling or riding a stationary bike, but in many ways, it’s a far more intense workout—and one of the easiest to overdo.

First, there aren’t many (if any) breaks in spin class. “When you’re biking outside, you have to be aware of road dangers like water and cars, so you have to slow down at times,” says Dr. Maureen Brogan, an assistant professor of medicine at New York Medical College who has conducted research into spinning. Especially if you’re a novice road rider, it’s going to take some time before you’re comfortable enough on two wheels to really push yourself hard for long distances. That’s not the case on a spinning bike, where newbies can hop on and ride hard from the start.

Popular spinning studios like Flywheel and SoulCycle have their riders clip their feet into the stationary bikes. As long as the wheels turn, legs keep pumping. Combine this always-working aspect with the thumping music, enthusiastic instructors and energetic group atmosphere of most spinning studios, and it’s easy to get intense exercise and burn calories by the bucketful.

“The muscles you use on a spinning bike, the gluteus maximus and the quadriceps, are some of the largest in your body, so you’re using a lot of energy,” Brogan says—600 calories an hour, and sometimes more.