Exercises that involve torso rotation are great for the obliques. This exercise usually requires a bit of a learning curve, especially for those who are new to working out. Before starting this exercise, try it without resistance. Introduce the medicine ball when you are comfortable with the basic movement.

Begin this exercise with the upper back resting on the ball. The feet should be wide and the hips high. Hold the medicine ball above the chest. With the arms straight, rotate to the side until the arms are parallel to the floor. It is okay to maneuver on the ball. The abs should be "sucked in" when performing this exercise. Keep the chest high.

Note: This is not an arm exercise. Try to focus on rotating the upper body using the waist. The torso rotation exercise can also be done alternating between sides (e.g. one sweeping 180-degree movement), although it is generally performed one side at a time.