Aaron Small, Master Personal Trainer Athletic Club Northeast: Gym Smarts: Lower Body Knee Extensions This is the knee extension machine which is use for building quadriceps strength. Sometimes we will adjust this machine so you can't do the last part of the range of motion in order to protect the knees. Some people have knee injury or soon out of a knee surgery what we want to do is prevent the last 30 degrees of the range of motion in the knee extension to protect the knees. So from here what you will do is extent your knees by bringing your feet up. The last part of the range of motion is locked. It is at this point, if I would go any further would cause the most pressure on the knees.