The best exercises incorporate more then one muscle group. This exercise works both your hamstrings and your core muscles. Moving pictures allow you to see the full range of the exercise.
Begin by laying on the floor on your back, arms crossed over the chest, with your heels on the exercise ball. Lift your bum off the floor into a bridge position. Lift one leg towards the ceiling.
Using the leg that it still on the exercise ball, slowly flex the hamstring, pulling the exercise in toward you. Hold for a few seconds, then extend your leg back to the starting position. Do about 5-10 reps of this to begin, and then switch legs (source).
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