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Rob Kardashian is headed back to the gym.  

The E! star generated a Twitter frenzy two weeks ago after being photographed noticeably heavier en route to sister Kim Kardashian's wedding festivities in France. Since, Rob has taken to social media to defend himself, and is now turning to celebrity trainer Gunnar Peterson (who has worked with Sofia Vergara, Angelina Jolie and Jennifer Lopez) for some help with his workouts. E! News got the exclusive from the fitness guru (who also works with Khloe Kardashian) on his plan to get the 27-year-old back into tip-top shape.

So how will Peterson get Rob to shed those extra pounds? That's all a matter of turning anaerobic exercises into and aerobic circuits. For you non-fitness gurus, that means doing fast-paced, multi-joint exercise in succession, no breaks allowed. The Beverly Hills-based fitness pro gave props to Rob, calling him, "an athlete trapped in a celebrity body."

He also praised his work ethic. "Rob is such a solid guy, when he gets in here. I don't care if he weighs 50, 100 or 200 pounds," Peterson told us Monday. "He works like an athlete. He never tries to beg out of a set. He never tries to hop off the cardio a minute or two before. He goes the distance every time."  

Watch the video now to find out all the details on Gunnar's exact fitness plan for Rob.

Don't miss the new season of Keeping Up With the Kardashians, premiering Sunday, June 8, on E!