The leg extension is a great quad-isolation exercise that can be performed with both legs simultaneously or with one leg at a time.

To prepare for this, adjust your back support so you are upright, and your knees align with the end of the chair. The pad in front of your legs should be placed over the top of the foot in front of the ankle and should comfortably roll while you extend your legs. If you find the pad rolling forward, place it slightly higher. If the pad is rolling backward, lower it.