Growing up in Columbus, Ohio, Chantha Thach always hated her thick thighs. The 29-year-old fitness apparel designer says she was a chubby kid and vividly recalls a childhood crush who constantly teased her about her size.

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A couple of decades later, a lot has changed. Thach, who now lives in New York City, loves her strong legs for helping her through the long and rigorous journey to becoming a female bodybuilder.

"I'm happier than I've ever been in my life," Thach tells me, sitting in the lobby of a Crunch gym in Midtown where she works with a trainer every day. Thach has dyed red hair and an impressive physique. Glutes bulge from spandex shorts, abs pop in three dimensions, and her ripped arms are constantly, unintentionally flexed: "This has given me the mental empowerment to be happy with who I am and to know that I can constantly improve myself."

This weekend, Thach will be one of hundreds of women to compete in the Brooklyn Grand Prix, the National Physique Committee's East Coast bodybuilding competition. While most people conjure up images of Arnold Schwarzenegger-like men when they think of bodybuilding, it's a sport that's become increasingly popular among women, many of whom participate in order to combat obesity, fight eating disorders, or just get into the best shape of their lives.

"In the early 2000s, everyone wanted to be a runner. These days, everyone wants to be a bikini competitor."

Bodybuilding—which entails resistance training plus a very specific diet that results in extreme muscle development—has been a fairly widespread men's sport for the better part of a century. Competitions arrived on the scene as early as the 1940s, but gained traction in the mid-'60s after the International Federation of Bodybuilding and Fitness and Mr. Olympia organized the first large-scale contests.

There were women who competed in these early events, but the sport only really began to resonate with female participants in 2001, when the NPC introduced the "figure division" and more recently, in 2011, the "bikini division." These categories still promote the prototypical bodybuilding image, albeit with a more feminine focus.

Bikini, specifically, was introduced to appeal to women who didn't want an excessively muscled body; judges in that category look for a physique that "is realistically toned, that a stay-at-home mother could attain and maintain," explains Christine Hronec, a Pennsylvania-based bodybuilding competitor and coach. Athletes are split into groups based on height and judged on features like small waists, toned shoulders, lean abs, and good posture.

"'Figure' still has that masculine type of look, but 'bikini' is what blew up bodybuilding into the next biggest sport," notes Hronec. "A beach body is always the one that's most desired, because abs are always in style. Training for the bikini category has opened up a lot of doors in the industry. In the early 2000s, everyone wanted to be a runner. These days, everyone wants to be a bikini competitor."

Coinciding with the rise of extreme fitness activities like CrossFit and obstacle course racing, bodybuilding is yet another example of a niche movement on the upswing. Hronec believes that the sport is particularly alluring because visible results don't take terribly long to achieve: "For the average active woman who is at a healthy weight, it can take 12 weeks to get fit." Plus, there's the empowerment aspect of it all—these days, men are no longer dominating weight rooms at the gym.

"Strength training is hard and challenging, it's something you actually have to go out and earn," Hronec adds. "Fitness is a status symbol these days, and bodybuilding allows women to have higher confidence once they attain a physique they feel comfortable competing with."

"The 'strong is the new skinny' movement is definitely taking over."

As Cassie Smith, an associate editor at BodyBuilding.com, explains, "More women are tired of the same skinny, big-boobed look in magazines and on TV. The 'strong is the new skinny' movement is definitely taking over. It's even in style to have a butt these days, and you can't do that by running. You have to weight train."

The Idaho-based site Smith works for has had a women's vertical for a few years now, but rising traffic—the vertical has seen a 151% growth over the last year—prompted a big overhaul a few months ago. It now publishes five female-centric pieces every day.

"Women are recognizing that lifting weights is good for muscles, bones, and joints," she continues. "It's good for your heart and your brain and helps you sleep better and live longer. It's a good life-altering thing to get into."

Transforming the body takes a tremendous amount of discipline and determination. Thach does cardio twice a day, every day. She'll sprint for 30-second intervals on the StairMaster for an hour and then speed walk on the treadmill at a 15% incline. Her weight training consists of bicep curls, squats, pull-ups, and deadlifts, increasing the weight with every rep. Her workouts at Crunch always manage to attract an impressive group of spectators who watch in amazement as Thach presses through her routine; a few have said they use her as inspiration to push themselves.

Thach, who went from weighing 165 lbs to 136 lbs at 5'5", says she couldn't train properly without simultaneously consulting a nutritionist. She's on a strict diet to "eat clean," and though she's not making anything complicated, her food prep is certainly time-consuming. She spends every Sunday cooking for the upcoming week and then separating her food into individual Ziploc bags to carry around for the next seven days. Her meals consist solely of protein and vegetables—no carbs or fruit.

"You can't be on top of your diet like this your whole life,"

Kyedi Thomas, a 33-year-old aesthetician and bodybuilder currently in nutrition school, says that the nutrition aspect is what initially got her interested in professionally competing.

When she was 19, 5'7" Thomas weighed 295 lbs. Her weight fluctuated over the years, and when she took a job in a stock room at a retail company, she lost 60 lbs just by walking up and down the stairs. She changed her diet, bought some exercise DVDs, and finally got under 200 lbs in 2012. Last year, a laser hair removal client suggested she try bodybuilding; one month in, she went from a size 10 to a size 6.

Thomas, who, like Thach, will also compete at the Brooklyn Grand Prix in the figure division, trains in the gym six times a week, but admits dieting is the hardest part.

"You can't be on top of your diet like this your whole life," she says, "but after the competition, I'll have learned a balance. I'll eat clean, but won't be as strict. Who I am right now is not the person I was a year ago. I love the way I feel, and there's no going back."

Rebecca A., who requested her last name not be used, trains at New York City's Steel Gym; Saturday's event will mark her second time competing in the bikini division. As someone who's struggled with anorexia most of her life, Rebecca calls bodybuilding "lifesaving."

"I think everyone has an unhealthy relationship with fitness and food at some point in their lives, and this sports gives you the tools to achieve your goals in a safe and meaningful way," she says. "The training and diet produces the body you've always wanted, and the sport has a give-and-take. It burns to train, but I know it's truly for my own benefit. Even if I don't win anything at the competition, I've already won just by the way I've been able to transform my body."

Like all extreme sports, bodybuilding comes with its dark side. Hronec notes that many female competitors struggle with body dysmorphia, and since the end to the sport's means usually involves a competition, its athletes tend to be hypercritical.

"Even if I don't win anything at the competition, I've already won just by the way I've been able to transform my body."

"A lot of women struggle with being hard on themselves, and sometimes they don't know the line to draw when comparing themselves to others," she says. "The sport isn't just about competition, it's about being secure with yourself. When a woman pursues this path, they need to do it for themselves and not base it off the affirmation of a panel of judges. They shouldn't seek outside approval."

The cost is also something competitors must consider. Bodybuilders pay for trainers, nutritionists, posing coaches, food, and dietary supplements and vitamins. When you add in competition swimsuits and shoes, travel and beauty expenses (most women get spray tans, as well as their hair and makeup done; some even get breast implants to help them compete), athletes can easily burn through $5,000 for each contest.

Not surprisingly, bodybuilding communities thrive on social media. Athletes use hashtags like #fitfam, #bikinicompetitor, #girlswhoflex, #shesquats, #eatcleantraindirty, and #fitchicks. Instagram is home to bodybuilding stars like Dana Linn Bailey, Cally Clarice Breaux, Michelle Bishop, Amanda Latona, and Ashley Kaltwasser.

Marianne Dabir, a 25-year-old styling assistant and bikini competitor, ascribes much of bodybuilding's rising popularity to Instagram, as well as platforms like Pinterest and Tumblr, all of which have strong "fitspiration" or "fitspo" communities. She says it's easy to understand why women in the bodybuilding world want to connect to each other: With rigorous workout schedules and regimented diets, it's hard for non-bodybuilding peers to relate to their experience.

"There's a big misconception that women who build muscle will bulk up like men."

"There are people who become introverted and resign themselves from the social scene because it's really hard to go out with friends and turn down cocktails and hors d'oeuvres," she says. "My group of friends are accommodating and are okay with me ordering sparking water and lime or pulling out my Tupperware while we're out eating. The biggest challenge for me personally is figuring out how to get all my meals down every two to three hours when I'm constantly on the run or on set for work—I have to figure out a new strategy pretty much every day!"

Dabir also believes social media breaks down stereotypes about the sport, mainly that female bodybuilders end up looking like The Hulk.

"There's a big misconception that women who build muscle will bulk up like men. It's actually not that easy," she laughs. "I wish I could just gain ten pounds of muscle! But I felt the same way before I started seeing photos of what it can do for your body. It doesn't bulk you up, but it gives you a nice butt and a small waist."

Despite the money, sweat, and tears it takes to prep for competition, many bodybuilders consider the lifestyle sustainable—to a certain extent, anyway. Not everyone trains as intensely in between events, but instead turn to personal training or a career in nutrition, like Thomas. Others dabble in modeling for fitness brands and supplement companies.

"Abs are a fashion accessory that everyone wants to have."

Unlike obstacle course racing—which, judging by its low return rate and messy economics, looks like it wont have a whole lot of longevity—many think this is only the beginning of bodybuilding's popularity among women.

"I do think this is here to stay, because sustaining the look is easier than getting to it," Hronec says. "Also, there's something about getting into bikini shape that is a perennial type of thing that all women go through. Abs are a fashion accessory that everyone wants to have."

The weeks leading up to a competition are the most crucial: Training is increased while calorie intake is decreased. One recent weekday morning, just a few days before Saturday's big event, Thach is at the gym, panting on the StairMaster. She takes a quick break to wipe her face and look around the room. She's feeling good.

"Life after the competition? I think I'll prep my food, but probably have more cheat meals and won't be as intense about it. I definitely won't overindulge." Thach stops to smile. "My mom hates this. The last time I went home, she asked me if I was a lesbian. She was like, 'You've been single for two years, your hair is dyed, you have no boobs, but you have muscle.' But I want to continue—I'm happy with the lifestyle."

Editor: Julia Rubin