The body’s joints are the hub of limb movement.  There are two lines of defense around the joints: the inner ligament, which is used to keep the trajectory of the joint movement normal, and the external muscle tendon, which is used to ensure the stability of the joint movement.  In the case of the knee joints, their line of defense is the posterior femoral muscle group.  Both inner and outer lines of defense cooperate with each other to protect the joints and keep them in their normal positions.