EXCLUSIVE: Why Jessica Alba & Allison Williams Love Trainer Kayla Itsines' 28-Minute Workout

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ET found out how Instagram's most famous trainer is inspiring people to get fit with her 28-minute bikini body workout.

Twenty four-year-old trainer Kayla Itsines counts Jessica Alba and Victoria's Secret model Candace Swanepoel among her millions of followers who have helped start a fitness revolution.

The Australian trainer began gaining attention by posting before and after photos of her clients' transformations. She reached another level when her boyfriend, Tobi, suggested she start posting videos of her workouts so that her followers could exercise with her.

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Now, Kayla has a legion of fitness fanatics who are proud to represent #KaylasArmy. She dished to ET's Jennifer Peros about her high-intensity workout.

"It's 28 minutes, and its circuit training," Kayla said. "You've got four exercises, and you've got seven minutes to do those four exercises as many times as you can."

With a fitness app called Sweat With Kayla and subscription workouts, Kayla has more followers than celebrity trainers like Tracy Anderson and Gunnar Peterson, and she's traveled all over the world as a part of her bootcamp world tour.

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Watch the video to see examples of some of Kayla's most effective exercises that you can try at home.