Want to know what an ahead-of-the-curve fitness phenomenon looks like? Kristi Molinaro’s 30/60/90 is it.

Molinaro created the High Intensity Interval Training class in 2005, long before HIIT was a buzzword (and probably contributed to making it one).

Nearly 10 years later, it’s still one of Equinox’s most popular, routinely-packed classes—in both New York and Los Angeles. In addition to Molinaro, there are 35 certified 30/60/90 instructors currently teaching it, DVDs for home workouts, and classes at New York Health & Racquet and 24-Hour Fitness, too.

On a recent Friday night at Equinox Greenwich Avenue in the West Village, regulars arrived a solid half hour early for class to claim their tiny section of floor space and then expertly negotiated space throughout, happily moving their planks to the right or left so as not to kick their neighbors. Fifty or sixty people in that time slot is common, Molinaro says, and she teaches two sessions back-to-back to fill demand.

“One of the biggest complaints I get is that it’s too crowded,” she says, “and that’s a pretty good problem to have.”

So what’s all the fuss about?

Like the name suggests, the class is made up of fast-paced 30-, 60-, and 90-second intervals, using a step, hand weights, and sometimes a medicine ball. The intervals, packed with plyometrics, strength training, and cardio, start steady and end with 30 seconds of jacked-up speed, which pushes you to the point of exhaustion almost every time.

Unlike Tabata, there are no breaks at all (unless you need them). To keep you from keeling over, Molinaro follows intense cardio moves like squat jumps with ones that give you a slight aerobic break, like push-ups or bicep curls.

Innovation accounts for some of 30/60/90’s success: “I think it was just so revolutionary in the beginning. When I created it, I wasn’t building it off of anything else that existed,” she says. Her inspiration came from results she was observing with the personal training clients she put through boot-camp-style paces, combined with a Times article on interval training that sealed the deal.

But while the 45-minute metabolism-stoking sweatfest is the main draw, the energy in the room is what really hooks her devotees. During my class, the music was booming with shouting and cheering throughout, and a contagious sense of camaraderie and fun—more than I’d gotten in a boutique fitness environment in a while.

“It’s going to sound corny but I feel the most proud…of the friendships and community it’s created. People really feel a certain ownership of the brand. They promote it with no incentive. We have parties, we do events, everybody is really connected,” she says.

The truth: When I initially raised my hand about being new, an obvious regular next to me told me not to worry, to “just go for it and I’ll move out of your way.” Afterward, he extended his sweaty hand for a fist bump and said, “Nice work!” which made me think, “I should get here early next week to claim this same spot…” —Lisa Elaine Held

For more information, visit www.equinox.com and www.306090fitness.com