To perform the Stationary Lunge w/Med Ball on Disk, start with one leg bent at a 90-degree angle with the shin resting on the ball. Your other foot is planted on the disk. Position your standing leg slightly further forward so there is some space between your working leg and your rear leg. Slowly bend the working leg and at the same time guide your rear leg backwards. Lower yourself as far as you feel comfortable and then push up with your working leg until you've reached the starting point. While pushing with the working leg, you should pull the stability ball back towards your body making sure to keep your leg firmly on top.