[Note that eNotes answers are intended for study purposes only and are not intended as midical advice. If you require medical advice, consult a health care provider.]
Physical therapists vary knee extension exercises to avoid putting stress on ligaments and to build additional strength. In the initial stages of therapy, immediately post-surgery or post-injury, it's important to make the exercise easier to avoid straining the ligament. Perform the exercise seated in a bed or floor with the back supported against a wall and a pillow under the knee, so that the extension is done only through a short (terminal) arc ("terminal knee extension"). Gradually increase the range of motion as the patient becomes stronger.
As the patient becomes stronger, it is possible to add ankle weights, and eventually use knee extension machines to provide additional resistance.