Reese Witherspoon, Naomi Watts and other celebrities love the cardio and HIIT classes at Burn 60 in Brentwood, California

In between filming movies like A Wrinkle in Time that take her all over the world, Reese Witherspoon is a regular at Burn 60 fitness studio.

The busy star — plus other celeb fans like Naomi Watts — joins the group classes at the Brentwood, California gym as often as possible for their signature workout, which combines treadmill cardio with circuit training.

“It’s a smart way to get the most bang out of your buck if you have limited time, because you spend half of your time doing strength training work with resistance bands and dumbbells and barbells and body weight, and half of the time on the treadmill getting your cardio in,” trainer Keith Anthony tells PEOPLE. “So it’s really for the busy professional or busy mom trying to squeeze a workout in.”

Reese Witherspoon Keeps In Shape
Reese Witherspoon leaving Burn 60
| Credit: FameFlynet

Some Burn 60 classes are mostly running — perfect for fans of the sport like Witherspoon, who’s often spotted on a run by her home — while others incorporate more of Anthony’s favorite circuit movements.

“I LOVE a reverse lunge with a dumbell, because it targets the glutes so well and gives you a nice stretch in the hip flexors. That’s one of my go-to strength movements,” he says. “I also love combo movements for the arms like a bicep curl to a shoulder raise to an overhead press to a tricep kickback. Your arms just get smoked.”

Celebrity Sightings In Los Angeles - March 02, 2017
Reese Witherspoon
| Credit: Bauer-Griffin/GC Images

One thing that Anthony, who has been at Burn 60 for over a decade, is particularly proud of is how well he can shape the workouts to his clients.

“Everyone who comes in the door I know because they’ve been coming for such a long time,” he says. “I know what their fitness goals are and their injuries and their bodies. It’s a group fitness workout, but I can really tailor it to each person.”

RELATED VIDEO: Try Carrie Underwood’s Three Go-to Workout Moves

But because it is a group class, Anthony makes sure that everyone is held to the same standard — and the celebs do not get any special treatment.

“I don’t tailor the workout to any of the celebrities in the room. The workout is tailored to whoever is in the room that day,” he says. “The movement that’s best for the famous people are the movements that are best for the moms who are bankers and landscapers and full-time moms. It’s the same movements. There’s no secret sauce.”