Results ... Kourtney Kardashian has been hitting the gym. Picture: Bauer-Griffin/GC Images Source: Supplied

And now her older sister Kourtney has showed off her own body transformation - complete with washboard abs and ripped arms - after her third child.

Kourtney, 36, gave birth to son Reign Aston in December - her third bub with long-term boyfriend Scott Disick - and has been working out with celebrity trainer Don Brooks to get back into shape.

She flaunted the results on Instagram, wearing a tiny black crop top and showing off her toned abs and arms in a mirror selfie.

Hard work ... Kourtney Kardashian is looking fit and toned after baby No. 3. Picture: Bauer-Griffin/GC Images Source: Supplied

So how did she do it?
Brooks told E! he uses the Don-A-Matrix method when training Kourtney.

“We train anywhere from three to five days a week depending on what projects are coming up. (Workouts last) anywhere from 45 minutes up to an hour,” Brooks said, according to E!.

So what does it take to work out like a Kardashian?

Killer curves ... Kourtney Kardashian, right, with boyfriend Scott Disick in Las Vegas last month. Picture: David Becker/WireImage Source: Supplied

Kourtney’s regime is split into four “high-octane” quarters (like a sport).

“The plank is a favourite, but we take it to another level opening and closing the legs like scissors while at the same time raising and dropping the hips,” Brooks told E!.

“So not only are you strengthening the core but also timing the hips, thighs and waist.”

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