Interest in fitness might be at its peak: A record 66 million Americans used health clubs last year, according to the International Health, Racquet & Sportsclub Association, while countless others work on getting in shape in other ways.

Some prospecting entrepreneurs have been all over this trend, using innovative new business models to turn passion for exercise into thriving brands with venture backing and millions in annual revenue. Here are six fitness empires that were built from the ground up, initially without any deep-pocketed corporate partners.

The brainchild of three fitness fiends looking for a new way to work out, Zumba combines aerobics with several types of dance, including salsa, hip-hop and mambo. The co-founders launched the brand in 2001 and began, for a fee, teaching instructors moves they could then relay to their students. In the following years, the idea took off: The company's revenue is not disclosed, but grew by 4,000 percent between 2007 and 2010, according to Inc. Now an estimated 15 million people practice Zumba in 180 countries each week. The workout is taught in 200,000 locations, including hospitals, schools and community centers. Instructors — there are more than 100,000 of them, by some estimates — pay a fee to get licensed, then a $30 monthly subscription to receive videos with new music and choreography. The company also has an apparel line that's been reported to sell 3.5 million units per year.

It took several pivots for Payal Kadakia to get the model for ClassPass right. Once a search engine for fitness classes, the company now offers monthly subscriptions that let customers sign up for classes at multiple studios. To date, users have made more than 35 million class reservations — from cycling and yoga to dance and Pilates — at 8,500 facilities in 39 cities worldwide. The company has secured $173 million in funding since Kadakia founded it in 2013 and was reportedly on pace for more than $100 million in revenue in 2016, according to Business Insider. In its Series C funding round earlier this year, ClassPass was valued at $470 million.

Mud. Water. Live barbed wires. These may sound like a nightmare to most people, but to some they're an invitation. Joe de Sena founded Spartan Race, a series of grueling obstacle-course events, in 2010, with the first race in Vermont attracting 1,500 participants. Like Tough Mudder and Warrior Dash, the endurance test has gained a ton of traction in recent years. I Spartan Race does not make revenue figures available, but entries to many of the races start at $100, and more than a million people will enter one of the 200 events in 30 countries the company will host this year. Overall, more than 5 million people have participated. That's a lot of folks who love pain.

You remember the craze: VHS tapes featuring Blanks leading fitness classes, yelling stern-yet-motivational phrases over fast-paced music. Tae Bo, which Blanks created in the late 1970s by combining elements of taekwondo and boxing (hence the name), began as a regimen Blanks taught in his own fitness classes. Years later he created videos based on the workout, which, thanks to late-night infomercials and endorsements from celebrities like Paula Abdul and Shaquille O'Neal, became a sensation. Blanks sold more than 1.5 million video sets in a one-year span from 1998 to 1999, grossing upward of $80 million. Along with videos of a new workout, Billy's Boot Camp, his total sales to date are well north of $100 million. Today Blanks teaches high-end classes at a gym in Sherman Oaks, California, where Tae Bo madness first began.

What began as a single fitness club in northern California is now, by membership, the largest privately owned fitness chain in the world. After injuring his knee in 1983, Mastrov relied on Nautilus machines at a local health club for his rehab. When the owner sold the club, Mastrov invested with the idea of turning it into an around-the-clock operation. What was then called 24 Hour Nautilus grew quickly, and a decade later the company bought out several of its rivals and rebranded as 24 Hour Fitness. The 18,000-employee company now has more than 3 million members at 400 locations worldwide. Mastrov sold his share when the company was acquired for $1.6 billion in 2005. He's currently CEO of Fitness Holdings Worldwide, a company with investments in gyms around the world, and is part-owner of the NBA's Sacramento Kings.

Michaels rose to fame as a personal trainer for contestants looking to shed pounds on The Biggest Loser. Since the reality show's 2004 debut, she's parlayed her fame as a notoriously tough coach into a massive fitness enterprise. She's released a series of workout DVDs, such as Bodyshred and Body Revolution, authored a handful of bestselling books about weight loss and in 2014 launched a women's and children's activewear brand called Impact. Her dietary supplements are on shelves at major supermarkets, and the Jillian Michaels app helps users create customized workouts (with Michaels' coaching) for a monthly fee.

— By Kevin Ryan, Inc.com