What’s the latest fitness craze? Forget going to fitness classes for the exercise – people are showing up for the music. In fact, plenty of people are choosing exercise classes based on how much they like the instructor's taste in tunes.

And fitness companies are responding by literally getting into the music biz. A lot of companies involved in yoga, Zumba and Spin are promoting new artists. And having DJs, just like at a dance club, to come and spin songs in class. They’re even licensing music from record companies so they can create and sell their own customized CDs and DVDs. So, their fitness clients can do the workouts at home.

Take Zumba Fitness, for example. They’ve has sold more than one million downloads of original songs, and up-tempo remixes on iTunes. Zumba fans even helped revive the career of rapper Vanilla Ice. The new Latin-flavor version of his hit "Ice Ice Baby" has sold nearly 17,000 copies.

Fitness companies are even sponsoring classes and festivals with DJs and live music, like one Zumba dance convention – where 8,000 participants dance to live music performed by artists like Wyclef Jean. But beyond the live events and CD sales, fitness studios and gyms are finding that if they play the right music, people will keep coming back – and start bringing their friends.