Bring yourself to the ground and enter a high plank position with your hands directly under your shoulders and your feet directly behind you, resting on the top of an exercise ball. Proceed to lift your left foot from the ball and tap it gently on the floor. Return the left foot back to the top of the ball and proceed to perform the same movement with the right foot. Make sure to keep your core contracted throughout. Keep performing this alternating pattern for the required number of reps (or time).