To paraphrase Newton's first law of motion, an object will continue to move until it's stopped by an external force. At Zumba Fitness, that external force is Ann Guzdial, an experienced Zumba instructor who's licensed to teach Zumba, Zumba Toning, Zumba Kids, and Zumba Kids Jr. classes. Until she tells her students to stop, they stay in motion—unlike malfunctioning Energizer bunnies—dancing to the pulsating rhythms of upbeat Latin music. That continual movement helps improve coordination, balance, flexibility, and endurance. But Guzdial is no drill sergeant. As she notes on her website, her classes feel like a party.