Nina Agdal in VIMMIA. Photo courtesy the brand.

Tone House, one of the many boutique fitness fads to hit New York in 2015, stands out from the rest of the candlelit cult mind-and-body cleanses that took the city by storm. Not only is it much more difficult, but it's also about building body muscle, rather than slimming down, hence the name. Classes, which run around an hour long, are an intense mixture of military boot camp and pilates, with the soundtrack of SoulCycle. Unlike cycling, where it's a race either against yourself or those around you, this workout is designed for a team. There's chanting and clapping and stomping—even call-and-response routines in a circle. So, leave your juice cleanse at the door. You'll be pounding protein bars before, during, and after class.

On a recent Tuesday morning, model Nina Agdal demonstrated to a crowd of around 10-15 journalists the benefits of Tone House at its only studio in Union Square. She started coming to class a couple of months ago. "When I first started, I was dying. But now I can get through it," she said. She's since become a regular, leaving SoulCycle in the dust. "SoulCyle didn't make a big difference on my body, I think because it's so stationary. Tone House is a lot of different movements and cardio. You can never come in here and not give it your all, because you will never get through it."

After everyone at the studio changed into Vimmia apparel, Agdal started clapping. It was time to let the toning begin.

"This is going to be really hard," Agdal said in her Danish accent with a big smile. Her abdominal muscles were evidence enough that the "tone" part was real. The "hard" part was apparent immediately upon walking in the door: the studio is lit with red bulbs, like you're about to be developed in a dark room, and bungee cords of every shape and size hang from the walls. The word "torture," comes to mind.

Another aspect of Tone House that differs from the rest is its warm-up routine: there isn't one. No deep breathing or time for motivational introspection. Instead, you're thrown right into it. Nina demonstrated first. She hopped over hurdles, sprinted across the studio, then got down on her hands to bear crawl. Let me tell you, there is nothing glamorous about a bear crawl, I don't care who you are.

The second half of the workout consisted of resistance training, more obstacles, and finally, ended with the group in a circle doing everything from lunges to push-ups.

"It feels so good when you're done," Agdal said afterwards, breathless and glimmering in sweat. In more ways than one.