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The gym

The coaching: Teachers are usually certified by Yoga Alliance, but since many gyms don't pay well, you may end up with a teacher who's certified but less experienced, says Timothy McCall, MD, author of Yoga as Medicine.

The classes: Most gyms provide props and a variety of classes, including yoga hybrids (though the benefits of pure yoga often get lost); atmosphere is less serene than a studio's. Then again, you get to hit the steam room after class.

The crowd: After-work classes average 25 to 40 people—usually without additional teaching help—and classmates may be less serious about yoga.

The cost: Usually free with gym membership.

Next: The rec center