Transcript for Boxing fitness craze gets women ready to rumble
In the old days, if you had told the woman she had the body of a boxer, it might not have gone over so well. But now that world class fashion models are gloving up, things are changing. We go toe to toe with some of New York's heaviest hitters. Are y'all ready, New York? We go in three, we go in two, we go right now, jumping jacks, let's go! It's the fitness craze sweeping the nation. It makes you, like, super confident. You feel kinda invincible. Reporter: The original work-out, once considered a man's sport, is catching on as the hottest way for women to get fighting fit. Everyone from celebrities to super models are stepping into the ring. It's fun. Especially when you're like really pissed at someone. Reporter: Kendall Jenner, Adriana Lima and Gisele sharing their boxing work-outs on social media. And jgigi hadid showing off her skills for Reebok. New boxing studios are opening all over the country. Rumble is backed by rocky himself. Rocky's got him on the ropes! Reporter: Models like Gisele and Gigi, people you would think would be off doing yoga, they're boxing. Why is this taking off like this? One, because it works. Especially coupling with the strength training. Reporter: Noah Neiman is the co-founder of rumble. Here, as many as 60 people train at the same time. There's nothing like a boxer's high. It makes you feel like Ali in his prime. Reporter: He says 70% of his clients are women. I've learned technique, and as I'm relaxed, I still feel invigorated and very, very powerful. Reporter: Two regulars at this studio are 24-year-old MARIA, and 19-year-old chase Carter. Both top models in New York City who say they use boxing regularly to stay in shape. What most surprised you when you started boxing? The strength of my core got super strong and leaned out in the middle, kind of like an hourglass. And everybody wants that. What do you think about phrases like punch like a girl? If we could just prove them wrong, right? Reporter: After a quick warm-up, it's time to hit the bag. Half the room starts on strength training, while the other half boxes. Noah says boxing works every part of the body, but legs are the foundation. Every punch starts there. The last thing we worry about is throwing our arms. We really want to generate that power from the movement of our body. Your knuckles are just an extension of your hips, your core, your legs, the most powerful parts of your body. Reporter: All this at a time when professional women's boxing has come to the forefront. These two led the charge for the U.S. At the 2016 olympic games. Mayer spends almost all of her time in the gym. Boxing, I mean, I just switched gears. It focused me, I stopped going out, I stopped partying. I cracked down in school. Reporter: Boxing is as much a mental work-out as it is physical. Women are invested in breaking boundaries and boxing has been, up until now, a male dominated sport. Women want to push that boundary. That's a very empowering feeling and women are into empowerment. You were saying it makes you feel more confident. Is it just the act of punching? I think it's the way that you hit the bag. When you learn to hit the bag in such a way that you can hear it, it's just so satisfying. Nearly 40 million people were practicing yoga tin the united States last year. Boxing glove sales are up 130% from last year. And boxing has overtaken yoga as the top search term on the site. It's for your mind and body. It's for everything. I like to lead with the mind first, but the body comes, trust me. And we want that? Yeah. ??? ??? Reporter: Back at rumble, 45 minutes later, the gloves come off. First of all, I'm absolutely drenched. You get surprised how into it you get when you're punching the bag. Because when you hit the bag, you can feel how hard you're hitting. I feel kinda tough now, like I can kick some butt. But for now, I need a break. Reporter: For "Nightline," I'm Diane Macedo in New York.
This transcript has been automatically generated and may not be 100% accurate.