Jennifer Aniston is "always hard working" when it comes to her fitness routine

She told E! News: "She really trains in all three of our signature formats: Strength Sculpt, Progressive Weight Training class and Sculpt and Burn.

"So, your strength, your mobility, as well as your cardiovascular health-she trains in it all, Keeping her practice has been truly incredible to witness firsthand.

"Anything that can challenge Jen's core is probably like one of my favourite things to try to do with her. Because she's so strong and so finding new ways to challenge her with the Pvolve method has been so fun as a trainer.

"I haven't given her anything yet that she said 'no' to. I'd say she is always hard working, always keeping me laughing with her humor, and it's a lot of fun. It's really a dream come true."

Jennifer - who shot to fame alongside Courteney Cox, Lisa Kudrow, Matt LeBlanc, David Schwimmer, and the late Matthew Perry in the classic 1990s sitcom 'Friends' - previously revealed she would be keen for her former co-stars to take up the method that combines low-impact functional fitness with resistance-based equipment.

She told 'Entertainment Tonight': "It's like I found a precious stone of some sort. It's just changed my whole outlook on working out. It's something I look forward to. I'm still trying to get Courteney to take a class with me, but I'm working on her. Lisa's working so often [that] I can barely get her in Los Angeles."

But the 'Morning Show' star insisted that her new exercise regime is not a "fad" and claimed that it goes back to the age-old principle of "calories in and calories out".

She said: "Fads are what they are - they're just fads. They come and they go, and it's not very science-based or backed by much. When I was in my 20s, was there a grapefruit diet for a second there? Sure. I remember doing that. I also remember doing Nutrisystem... Things like that just aren't sustainable.

"Calories in, calories out. You put good, healthy vegetables, organic foods in your body, your body is gonna thrive on it. Put crap in your body, and your body's gonna show that.

"Your body is just a reflection of everything that you do. It's about maintaining a balance.

"Eighty percent straight as an arrow, doing all that we're supposed to do, and then give yourself 20 percent of fun.

"That's like, let's go out with the girls and get Mexican food, margaritas. Let's have a pizza party. Let's barbeque burgers and fries, hot dogs, by the pool. It's indulging and enjoying yourself and maybe saying, 'I'm not gonna work out today."