The knee is a big hinge joint that is built to flex and extend. It is held together by 4 major ligaments: your ACL, PCL, MCL & LCL. There are also two C-shaped cartilage cups in the center called your meniscus that act as shock absorbers and help add stability to the knee. The knee cap is encased in the tendons from your quadriceps and can be a common source of pain. See the video for more detail.