Exercise Ball Jackknife

Start by assuming a push-up position with your hands on the floor and your shins resting on an exercise ball. Keep your body in a straight line from head to heels and your core engaged. Slowly roll the ball towards your chest by bending your knees and pulling them in, then use your core muscles to control the movement. Once your knees are near your chest, slowly extend your legs back to the starting position, keeping your body stable throughout.