The first thing you need to do before visiting the Manhattan studio In Fitness & In Health is to learn some new lingo. The word overweight is not spoken here. Neither is the word fat. There are large women or plus-size women. You won’t hear words like weight, diet, or calories, either. Rochelle Rice, the center’s founder, prefers words like holistic, spirituality, and healing. They’re not just euphemisms. They’re the whole philosophy.

Ms. Rice, 42, calls her fitness studio a “wellness center.” She offers classes in movement, yoga, fitness, and Pilates, as well as nutrition and cooking courses. She aims to help women get connected to their bodies, applying a holistic approach to weight and creating a space for women to feel comfortable. “Most of our culture is from the outside working in: take this pill, do this diet, do this exercise,” said Ms. Rice. “I work from the inside out.”

Ms. Rice, a lithe blonde who was once a professional dancer, could hardly be more different from Gary Heavin. Mr. Heavin, 49, a hearty businessman from Waco, Texas, is the founder of Curves For Women, In Fitness & In Health’s main competitor in the niche of fitness centers catering to overweight women.

And a formidable competitor it is. In Fitness & In Health has just one location on Third Avenue near Grand Central Terminal. Curves, by contrast, has 10 locations within 5 miles of Ms. Rice’s studio – and another 8,340 across the country.

Both founders agree that the national fitness industry appeals to a narrow cross-section of the population. Only 12% of people in the country belong to gyms, even though a solid 60% of the population is overweight.

And both believe that traditional fitness centers alienate plus-size women. “Other gyms see the fat on a person and that’s all they see,” Ms. Rice said. “Most gyms are looking for a certain type in their gym.”

“The equipment didn’t fit them and the men stared at them,” Mr. Heavin agreed. “It was an uncomfortable environment.”

They also agree that the solution is to offer a fe male-only environment where women don’t feel like they’re being judged by men. “Curves is designed for just women. The equipment fits their bodies, the instructors are women, and the other members are women,” said Mr. Heavin. “Ninety percent of Curves owners are women, and most were members before they were owners.”

Ms. Rice’s center is also women-only, but that’s where the similarities end.

At the core of their disagreement are radically different philosophies about what a women’s fitness center should be. Ms. Rice tries to provide a nurturing, healing environment that emphasizes building confidence and self-worth over losing weight. She believes it’s fine for women to be heavy, and encourages moving women toward their “natural” weight, even if that means staying plus-sized.

Mr. Heavin disagrees. “We have an obesity epidemic in this country, and two-thirds of Americans are overweight,” he said. “The data says that if you’re carrying excess weight, that you’re going to die six years sooner, and if you’re a woman, you’re probably going to die of a heart attack, or get diabetes, or have arthritis.”

At Curves, the explicit focus is on losing weight. “The reason we have almost 4 million members now is that we have an effective program,” Mr. Heavin said. Women who follow the program, he says, can expect to lose 10 pounds in 10 weeks.

By contrast, Ms. Rice’s approach is to take the pressure off by not even talking about weight loss. If women lose weight, great. If not, they’re still improving their lives by becoming more active and gaining confidence. “I want her to suspend for the one hour she’s here the idea of worrying about her weight,” says Ms. Rice. “Weight loss is a byproduct.”

Ms. Rice’s movement classes attempt to address the physical needs of plus-size women: getting down onto and up off of the floor, getting up again after falling down, getting on a bus, doing the laundry. Every exercise in class corresponds to a life activity. The overhead press helps with putting things on top of shelves. Squats are good for public toilets. Buttocks work helps with climbing stairs. Pilates exercises prevent injury when carrying groceries.

In Fitness & In Health values self-worth as a prerequisite for anything else. “On the inside she feels better, there’s an increase in self-esteem, movement begins, and things start happening,” Ms. Rice said. “Life starts happening.”

Mr. Heavin couldn’t disagree more. “If that were true, it’d be like telling somebody, you have to lose weight before you join the gym,” says Mr. Heavin. “Self-esteem is a consequence of achievement, and there’s not much substitute for that. You can’t sit around and do self-talk to make yourself feel better about that. You have to act on those thoughts – action is the key.”

Curves is based on a circuit-training regimen, in which women do 30-second stints on resistance machines, alternated with jogging in place and other aerobic exercises. Since the program is so simple, the gyms can be very small. As a result, it costs comparatively little to start a Curves franchise gym.

Mr. Heavin opened his first health club 20 years ago, when he was 20 years old. “Curves wasn’t an evolution, it was a devolution,” he said. “We took the common denominator from what was working at our first health club, which was the circuit, and we created a franchise.” The result, Mr. Heavin claims, is the fastest-growing franchise in history.

Maybe the difference in their approaches is best explained by the experiences that brought them to this point. Ms. Rice was a professional dancer who became bulimic. So in grad school, she studied plus-size exercise and discovered that there was very little material available on the topic. After graduating, she created a six-week program for plus-size women that emphasized healing, then founded In Fitness & In Health in 1990.

Mr. Heavin was guided by a very different experience. He woke up one morning when he was 13 years old to discover that his mother, who was 40, overweight, and hypertensive, had died in her sleep. At the time, he didn’t realize that the experience would drive his entire professional life. But at age 17, he was pre-med in college, and by age 20 he was running a women-only fitness center. At age 40, he had what he describes as an epiphany. “I was standing in a room teaching fitness and weight loss to women, which is what I’ve done for 20 years, and I caught myself scanning the crowd for the face of my mother,” said Mr. Heavin. “I realized what had driven me all of my life, and what my destiny was going to be.”

In Fitness & In Health, 555 Third Ave., 212-689-4558, www.infitnessinhealth.com.

Curves International, multiple locations, 800-848-1096, www.curvesinternational.com.