Julia Roberts has always managed to captivate the fans and conquer even her harshest critics with her amazingly bright smile. If you ask her, this is her staying power, as also is her almost unbelievable ability to always look the same and not pile on the pounds. Still, this is not something that comes effortlessly, as her personal trainer Kathy Kaehler reveals for InStyle magazine.

According to the trainer, Julia does work out a lot whenever she gets the chance, since she’s most of the time busy in between her acting career and being a full-time mom. Yet, the star is aware she needs to do something to stay in shape, despite still looking as gorgeous at 41 as she did when she was in her 20s. One of Roberts’ favorite workouts is, according to Kaehler, the aquatic type because it works the muscles while also helping her improve her strength.

“We change her workouts depending on the role. I try to keep it very creative so she doesn’t get bored.” Kaehler tells InStyle of her client’s routine. “She needs to look good on-screen, but she also needs the strength and energy to run around after her kids off screen. Swimming and water exercises are great ways to build up endurance and stay fit.” the trainer further explains. However, swimming and pool exercises are not all that Julia Roberts does to look this amazing, Kaehler reveals, as the star also sticks to a balanced, healthy diet and works out outside the pool as well.

Equipped with a flotation belt, Julia does cross-country skiing, running and arm exercises in the deep end of the pool at her Malibu home, says the trainer. A session starts with a few laps of regular swimming as a warmup, then Julia is strapped with the flotation belt and she runs 1-minute intervals at various speeds (slow, moderate and fast), repeating this exercise five times. As noted above, this is just one small part of the actress’ workout routine, says Kaehler.

One of Julia’s guiding mottos is that health must come before everything else. As the actress herself stated on previous occasions, she’s never the one to try to become too thin for her own good; therefore, she sticks to a healthy and balanced diet. She works out whenever she gets the chance and has chosen Kaehler to have someone to relate to, since she too is a working mother and can best understand her needs and the constant pressure for time she is under.

According to Kaehler, Julia Robert also mixes in strength training, Pilates and cardio with her aquatic workout, for a full-body routine meant to keep her fit and glowing with health.