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Kourtney Kardashian, Gym, Instagram

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Kourtney Kardashian is clearly putting in work to achieve a sexy, bikini-ready body. And the results? Well, amazing.

The hot mama-of-three has been taking to Instagram with evidence of her high-powered fitness routine, even calling out sister Khloe Kardashian's butt as her workout motivation (don't worry Kourtney, it's ours too).

So obviously we're just a little (read: a lot) curious about her third (!) post-baby body transformation. Turns out that Kourtney's bod has been whipped into shape by a routine called the Don-A-Matrix method, and sources tell E! News exclusively that she "swears by the program."

OK, we know what you're thinking now, but no—it's not a Fifty Shades of Grey-themed workout!

Kourtney Kardashian

T Maidana/AKM-GSI

Developed by Kourtney's trainer Don Brooks, the Don-A-Matrix method is "set up like a game" and involves Kourtney performing a variety of high-octane moves within the space of an hour (she trains with Don three to five times a week, currently). We caught up with Brooks to get the scoop on the E! star's staple routine, including her favorite moves and what she considers the ultimate weight loss test. Read on for what keeps her fighting-fit below!

Kourtney Kardashian

Instagram

So how did Kourtney first hear about you?
Through Fergie! I was on tour with the Black Eyed Peas and Courtney attended a concert while in NYC, and she loved how I had Fergie's body looking.

How often does she come in each week—and how long are the workouts?
We train anywhere from three to five days a week depending on what projects are coming up. [Our workouts last] anywhere from 45 minutes up to an hour.

What are some of the exercises you do? What sort of benefits does Kourtney see from doing them?
The plank is a favorite, but we take it to another level opening and closing the legs like scissors while at the same time raising and dropping the hips! So not only are you strengthening the core but also timing the hips, thighs and waist. [We also do] the trunk twist along with a back row using the bands. The benefit of this exercise will give you flat abs and coke bottle obliques while removing those fat handles from the back and arms.

Kourtney Kardashian, Scott Disick, Penelope Disick, Mason Disick, Mexico

Brian Prahl / Splash News

How would you describe the "Don-A-Matrix" Method? 
The Don-A-Matrix Method is set up to resemble a sports game format consisting of four quarters. Each quarter has 3 sets of two different exercises, combined with rest or relief periods after each quarter. 

Nice! But how is it different or better than other fitness methods? 
Don-A-Matrix is Rated R for Results! The method challenges the cardio and muscular endurance system to tone and sculpt the body while burning fat.

Speaking of, do you need any special workout clothes or accessories for this routine?
Don-A-Matrix training gear. We use the resist-a-bands a lot—the bands are very effective for toning without bulking or injury. 

What changes have you seen on Kourtney's body from working with this method?
Along with weight loss, her body fat percentage and fat mass has decreased tremendously and lean muscle mass has increased.

Kourtney Kardashian, Instagram

Instagram

Amazing. So is Don-A-Matrix more about toning or weight loss?
Both! Most people think it's just weight loss to achieve the desired physical goal, but toning while achieving weight loss will help you get to the physical goal faster.

Has Kourtney seen a big difference in body shape since she started?
Absolutely! Like most women the clothing test tells all—being able to fit in those jeans or shorts verifies the difference. So I love when she says [she's] now able to fit into her favorite jeans or shorts again!

How long has she been training with you?
We started training right after her second pregnancy. So around two years now.

What other celebs are into Don-A-Matrix training?
Fergie
, Josh Duhamel, Will.i.am, David Boreanaz and professional baseball player Matt Kemp.

—Reporting by Sara Kitnick, additional reporting by Ken Baker