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This exercise is also suitable for the strengthening of the core muscles but as it belongs to the more difficult exercises it is only suggested for advanced learners or in the presence of a helper for the sake of avoiding injuries. 

Place yourself in plank on the ball in such way that your elbow and forearm should be under the level of your shoulder on the ball. You should support yourself on the front of your toes with a straight torso in a straddle of shoulder width. Focus on straining your buttock, pulling in your belly and slightly overthrow the pelvis and you hold it… hold it… and hold the position firmly. Then you lift one your legs straight, hold it for 1-2 seconds then let it go back down to the ground. Repeat this with the other leg as well.