Alison Dougherty used to pay $30 a month for a health club membership. She’s much happier now that her fitness expenses average $500 a month. 

The 27-year-old New York marketing professional exercises at Title Boxing Club NYC. She spends $139 monthly for unlimited classes and $100 for weekly personal-training sessions. She says even though the boxing studio presents fewer options, the workouts are more fun and effective than her treadmill-and-weights sessions were at the gym.

“It’s definitely a bigger investment,” says Ms. Dougherty, who has curbed shopping trips and brought her lunch to work to afford her boxing-boutique investment. “But it’s been worth it.” She believes her intense pugilistic workouts helped her break two hours in the half-marathon.

Specialized fitness studios used to be pricey enclaves for a few fanatics. But the ranks of their fans have surged, and now stand-alone cycling studios, boot camps and ballet barre rooms are transforming the fitness industry. About 42 percent of the 54 million members of health and fitness facilities in the U.S. say they use fitness boutiques, according to data in an upcoming report by the International Health, Racquet & Sportsclub Association written by consulting firm ClubIntel.

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