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Leg drop:Lie on back with arms by sides, palms down, legs long with a stability ball between lower legs. Engage abs and lift legs, bringing ball directly over hips. Keeping lower back pressed firmly into floor, slowly lower legs toward the floor (try not to let them touch floor). Do 3 sets of 15 reps.
Abs Exercises: Swiss-Ball Leg Lifts - 10 Tricks to Amp Up Your Favorite Abs Exercises - Shape Magazine - Page 7