This joint complex is made up of three bones, the femur (thigh bone), tibia (leg bone), and patella (kneecap). There are two joints in the knee, the patellofemoral joint between the kneecap and the end of the thigh bone, and the tibiofemoral joint between the end of the thigh bone and top of the leg bone. Four ligaments stabilize the tibiofemoral knee joint, the medial collateral ligament (MCL), lateral collateral ligament (LCL), anterior cruciate ligament (ACL), and posterior cruciate ligament (PCL). Two disc shaped cartilage structures called the meniscus act as a shock absorbing cushion between the femur and the tibia. Articular cartilage is the smooth cartilage cap on the end of the bones in the joint which provides a low friction surface for maximum joint function. The knee joint extends (straightens out) when the quadriceps muscles contract, and flexes when the hamstring muscles contract. The quadriceps muscle and tendon attach to the patella (kneecap), which increases the lever arm and power of contraction of the quadriceps.