How Gwyneth Paltrow Got the Butt "She Always Hoped for"

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Gwyneth Paltrow admits it was an "arduous" process to get in shape for Iron Man 2.

The actress, 37, worked out for an hour-and-half a day with a combination of trainer Tracy Anderson's dance cardio and a specially-made "terrifically difficult leg series" video.

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"This 26-minute long video was an amazing start for getting the butt and legs I had always hoped for," Paltrow -- who gained 10 pounds of muscle to reprise her role as Pepper Potts -- writes in her GOOP newsletter. "This regime got me into the shape of my life and I continue to return to it when I have a specific event so that I can do ridiculous things like be a 37-year-old mother of two and wear shorts!" (She is mom to Apple, 6, and Moses, 4, with husband Chris Martin.)

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"Also, to really get in line, I followed her strict diet for 5 days to lose my extra winter weight," adds Paltrow of Anderson's recipe plan, which includes kale juice and super low-carb vegan wraps. (Read all the recipes here.)

"Now, I am not a good dieter and I cannot ever do it for long -- only when there is a goal in sight, which there was in this case," writes Paltrow, who ditched the strict eating plan to chow down on fried chicken and ice cream sundaes while filming her next flick, drama Love Don't Let Me Down.

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“She loves to eat. She pigged out,” Anderson joked to last week.