Seated oblique rotation is one of the best rotation exercises to work on oblique muscles and waistline. A medicine ball or exercising ring gives more challenge to your muscles in order to produce quick results. Keeping your back and spine in neutral position is the key to get maximum benefits of the workout. You can lift your feet up from the floor while rotating to add more difficulty in the exercise. Rotation exercises work on your shoulder muscles along with obliques to strengthen them when using exercise equipments.