Do stability balls actually work?

Personal trainer Matt Malotki replies: You can use stability balls for training without weights, but these balls can also be integrated into training with weights to enhance your gains and help prevent injury.
Weight training with stability balls has several advantages over more traditional weight training. For instance, when you do a set of dumb-bell presses while seated on a stability ball, you are forced to work your core muscles - your abdomen, hips and lower back - much more than you do when you're seated in a rigid chair.
You'll need to reduce the amount of weight you're lifting, especially at first because of these increased demands. If you use the balls for a few weeks, you will improve your stability and may even enhance your overall strength. When you return to your regular weight training, you may find you can lift more weight than you could before doing the stability work.
Using the balls may also start to change the appearance of your musculature. Often, it becomes more streamlined, more detailed and less blocky. When you sit on the balls, you're still taxing your body and target muscles fully, but you're challenging them to work differently from the way they do when your lower body is fully stabilised.
If you're interested in making your physique more defined, alternate four-week cycles of your regular weight training with four weeks of stability ball weight training. I've included some exercises for each upper-body part.
Shoulders - dumb-bell shoulder presses, seated lateral raises and front raises.
Back pull-downs and cable rows - seated on a stability ball. 'For pull-downs use a high cable and for rows use the low cable and pull the weight towards your chest,' Malotki says. 'You can perform either two-handed or one-handed versions.'
Triceps - seated one-arm triceps extensions and French presses using a cambered bar while lying on a stability ball. 'Beginners can use one dumb-bell at a time until they feel comfortable with balance and control.'
Chest - Dumb-bell presses and dumb-bell flyes. Malotki says, 'You can get a much greater stretch on the way down when you perform dumb-bell flyes on a stability ball.'
Biceps - seated alternating dumb-bell curls and hammer curls.