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Ellen Latham never thought her love of fitness would result in owning one of the fastest-growing businesses in the United States.

Latham, who owns a boutique fitness chain called Orangetheory, has snagged the No. 1 spot the annual ranking of the fastest growing women-owned or led businesses released by the Women Presidents’ Organization (WPO) and American Express on Thursday. Though this Orangetheory’s first time at No. 1, the Florida-based company is not a newcomer to the list; it was ranked No. 3 last year.

The list, which is in its tenth year, ranks companies based on both percentage and absolute growth. To qualify, businesses must be privately-held, woman-owned or led, and have reached an annual revenue of at least $500,000 as of Jan. 1, 2012. The 50 companies generated a combined $7.2 billion last year, up from $75 million in 2007, the first year the list was compiled.

Latham co-founded Orangetheory with just $50,000 from her personal savings in 2010. Since then, the business has flourished: In 2016, Orangetheory reported $451 million in revenue, up from $87 million in 2014— success Latham never truly expected.

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“We hit the ground running when we decided to franchise the company,” she says. “But I never thought it would explode to where it has.”

Orangetheory now flaunts 630 locations across the U.S., and has spent the last year breaking into the international market. “It’s been the biggest reward, but biggest challenge,” she says of expanding outside of the U.S.

Orangetheory is known for its metabolic training tactics—the roughly 60-minute, fat-burning classes are a blend of strength training and cardio, using exercise equipment like treadmills, indoor water-rowers, and free weights. Participants wear a heart-rate monitor, which is synced with monitors in the studio to see real-time readings.

“People are getting dramatic results in short period of time. That’s why the studios are jammed and we can’t open enough,” Latham says.

While the business on the list range across a wide variety of industries, there is one thing they—and the women who lead them—have in common, according to WPO president Marsha Firestone: innovation. “It’s definitely one of the characteristics that these women have that drive growth,” she says.

As for Latham’s advice to other aspiring female entrepreneurs, especially to those who have a great idea later on in life? “I was 53 when I started Orangetheory. So why not you?” she asks.

Top-ranked Orangetheory is followed by Pinnacle Group, an IT workforce solutions firm based in Dallas, which has remained in the No. 2 spot for the second year in a row. IT services firm Technology Concepts Group International, a newcomer to the list, was ranked No. 3. You can check out the full list of 50 below: