Jason Biggs was spotted at Barry’s Bootcamp in New York City’s Tribeca neighborhood last month (and even made a joke about it on Instagram). A few days later Jake Gyllenhaal and Ruth Wilson took another Barry’s Bootcamp class in the city — and supposedly followed their sweat session with a smooch.

So what’s the deal with this celeb fave?

“Barry’s is a results-driven workout,” partner and celebrity trainer Joey Gonzalez tells PEOPLE. “It’s for people who are totally serious about getting in shape and are frustrated with having tried things that just don’t work.”

And the love from celebs is nothing new. The boutique fitness class launched in 1998 in Hollywood, California, so “the whole culture with celebrities has been going on for decades,” says Gonzalez. “It’s just gotten more exposure as we’ve opened more locations. A few weeks ago I had six celebrities in one day in various New York classes.”

And the workout, which alternates between cardio and strength training in 15-minute intervals, keeps everyone coming back for more. “Cardio absolutely helps people get into shape. Aside from burning fat, it strengthens your heart and your lungs,” says Gonzalez. But that’s only half of what Barry’s is about. “The other 30 minutes is true strength training. You’re building lean muscle tissue and raising the resting metabolic rate so you’ll burn extra calories throughout the day.”

So you might start running on the treadmill at various speeds and inclines, and then switch to floor work with weights. And then you hit the treadmill again. And then back to the floor.

COURTESY OF BARRY'S BOOTCAMP

It’s that back-and-forth that makes the workout so effective, according to Gonzalez (above, in photo). “Agents and their clients know how well it works and how quickly it works. So when people have to get in shape right away for a scene or a photo shoot, we will see them five times a week for at least two weeks prior.”

It helps that the experience is tailor-made. “You can really customize the workout at Barry’s. Our weights range from 5-lb. to 70-lb. dumbbells. You’ll see some dudes crushing some major steel in class.” But that doesn’t mean women should be intimidated, he adds. “Our customer base is educated. The women have had that a-ha moment of ‘Oh, I should be lifting weights! This is okay to do. I’m not going to end up a meat head with huge, broad shoulders and guns just from doing 25 minutes of strength training a day!’ ”

And no, you don’t need a certain level of athletic prowess to succeed in class. “No matter who you are or what sort of background you come from, there’s a place for you. We’ll have one class with someone who is 60 lbs. overweight and right next to that person will be an Ironman triathlete, and next to that person might be someone in their 60s who can’t run and just walks on the treadmill. It really is for all age ranges, all body types, all levels of fitness.”

More from People Great Ideas:

Jennifer Lopez Gives Us a Peek into Her Workout Routine

Madonna’s Trainer Reveals How You Can Get Those Killer Arms

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