The knee joint is an intricate mechanism that functions as a hinge in the creation of all motion in the legs, as well as assisting in the provision of support by the legs to carry the mass of the upper body. The knee is also the primary support for the body when in a kneeling position.
The bones that provide the framework to the knee are the femur (thigh bone), the tibia and fibula (the bones of the lower leg), and the patella (the kneecap), which is positioned over the approximate center of the joint. As the knee is constructed as a hinge, there is little movement in either the femur or the lower leg bones in the function of the knee. The patella is constructed so as to glide along the femur; the patella provides protection to the interior of the joint from forces directed to the front of the knee.
Between the head of the femur and the heads of the tibia, the heads of all of the bones that meet at the knee joint are covered with a substance called articular cartilage, a slick, friction-reducing fiber that is present in almost all other joints in the body. There are two meniscuses, a type of cartilage that serves to both provide shock absorption to the joint as well to assist in the reduction of the friction that would otherwise occur when bones come into contact. As with most other joints in the body, there is a synovial capsule surrounding the joint, in which a viscous fluid, designed to aid in the lubrication and general flexibility of the structure, is present. This lubricant is the synovial fluid. The meniscus can become injured both through a gradual wearing down of the fiber, as well as through a ligament tear. The articular cartilage can also become worn through the accumulated stresses of athletic activity; the expression "bone bruise" is an injury to this cartilage lining on the heads of these bones.
The internal knee joint is contained within a network of ligaments, each which performs a specific stabilizing function. The tibial collateral ligament is connected to the two meniscuses present in the joint. The transverse ligament stretches from one side of the joint, below the head of the femur, to the opposite side. The patellar ligament connects the kneecap to the tibia. The fibular and tibial collateral ligaments assist in the stabilization of the joint through the connection with each of those lower leg bones to the femur. This ligament is often referred to as the patellar tendon; as it connects one bone to another, it is properly a ligament.
The most prominent knee ligaments from a sports science perspective are the anterior cruciate, medial collateral, and posterior cruciate ligaments. It is these ligaments that are most frequently injured in the course of sports, particularly those activities involving physical contact to the knee. The anterior cruciate, known as the ACL, is responsible for a significant degree of the stabilization at the front of the joint. The ACL connects the tibia to the femur, hence its importance in providing stability to the joint overall. Damage to the ACL renders any type of lateral or explosive movement impossible. The medial collateral ligament (MCL) runs along the outside of the knee joint, and it is the ligament that will receive a direct application of force with any contact to the outside of the leg. The posterior cruciate ligament (PCL) stabilizes the rear of the knee joint. These ligaments are vulnerable to injury not only from a direct force applied to the knee, but also from the forces created by sudden twisting or explosive movements.
The hinge effect of the knee joint is determined by its relationship to both the upper and lower leg structures. The knee is joined to both the gastrocnemius and soleus (calf muscles), as well as to the quadriceps and the hamstrings of the upper leg by tendons. It is these muscles working in concert that permit the virtual 180° extension of the knee. The hamstrings are of particular importance to the overall muscular balance of the knee joint; ACL injuries in particular will often arise when the hamstring structure is weak in proportion to the rest of the leg muscles.
Optimal knee function is essential to the basic movements required in virtually every sport. Whether the athlete is standing, walking, kneeling or crouching, running, jumping, skiing, skating, or cycling, each of these movements is achieved through either the flexion or the extension of the knee joint.