Her secret: Hip thrusts.

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Credit hip thrusts for getting Megan Fox back into her pre-baby shape. "They helped lengthen (i.e., flatten) her abs and hip flexors, while strengthening her hip extensors, back, glutes and hamstrings," says her trainer, Harley Pasternak.

Lie faceup on the floor with knees bent, heels on top of a bench, arms extended out to sides (A). Engage abs, squeeze glutes and slowly press hips up toward ceiling, forming a diagonal line from knees to shoulders (B). Hold for 1 count; lower back to start. Do 3 sets of 20 reps twice a week.