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To make this exercise more challenging, you can bend one of your legs as you lift your body off the ground. You can also point one leg up at the ceiling for an even greater challenge.
Bend it at a right angle as you stretch your body upwards, then switch legs for the next repetition. Just lift it so it's perpendicular to the floor as you lift your torso up, then set it down on the ball again and switch legs for the next repetition.