Knee: Genetic and Non-Athletic Conditions Affecting Performance

For sports that require effective running, jumping, or propulsion of any form, the knee is one of the most scrutinized parts of the anatomy. There are a number of conditions, unrelated to the stresses of training or competition, that may affect knee joint performance.

As a part of the living organism that is the human body, the growth and inherent structure of the knee is determined by the genetic makeup of every individual. Human genetics, as determined by the individual codes contained in the deoxyribonucleic acid (DNA), occasionally create unusual conditions affecting the development of the knee and its surrounding structures.

The knee is a joint with a unique construction. It is a hinge joint, and it is capable of only a very small range of rotation; it has a primary function in movement to flex (bend) and to straighten (extend). Created at the juncture of the femur (thigh bone) and the lower leg bones, the tibia and the fibula, and protected by a bony patella (kneecap), the knee joint is a complex construction of cartilage, tendons, and ligaments, all of which contribute to the stability of the structure during both movement and forces applied on contact.

The cartilage of the knee, known as the meniscus, is positioned to provide both shock absorption and a reduction of friction in joint movement. The long patellar tendon runs from the joint to connect the lower leg. The quadriceps muscle of the thigh is also secured by tendons to the knee. The hamstring muscles at the rear of the thigh and the gastrocnemius (calf muscle) are the muscles connected to the knee that power its flexion.

There are seven different knee ligaments, the strong, fibrous tissues that secure the bones of the joint; these tissues are essential to the stability of the joint. The best known of the knee ligaments, due to the frequency of injury in sports, are the anterior cruciate ligament (ACL), the medial collateral ligament (MCL), and the posterior cruciate ligament (PCL). The PCL is designed to prevent the femur from moving forward onto the tibia during movement; the ACL is constructed to prevent the femur from moving backward.

As a flexion and extension joint, the knee will not generally rotate on its axis. At times when the knee is bent, with the foot on the ground, there may be a degree of rotation in the femur as it acts in the knee joint. The tibia moves during a walking or running motion to permit an efficient foot strike.

There are many genetic conditions that may impact the efficiency and the stability of the complex functions of the knee, with a number of rare genetic bone and muscle diseases that affect all joints, including the knee, such as Marfan syndrome, a hereditary condition that alters both the elasticity of the tendons and ligaments of the body, as well as bone growth. Malfunctions of the pituitary gland and irregularities in the production of human growth hormone due to genetic reasons may cause abnormal joint growth.

There are four common genetic circumstances that specifically impact the health and development of the knee joint. These circumstances include:

• Growth plate development: Growth plates are the portion of the longer bones of the body where the length and the thickness of the bone are governed. A debilitating condition that may occur throughout the body is known as osteochondeces, which is caused when the normal progression of bone growth in a young person from cartilage-like cells to bone is disrupted. In some circumstances, the growth plate grows more slowly than the surrounding ligament of tendons.
• Osgood Schlatter disease (OSD): OSD is a commonly occurring condition in adolescents. Similar in cause to growth plate injuries, OSD arises when the patellar tendon pulls on the surface of the tibia to which is connected, but due to the differing rates at which the structures grow, if the tibia grows at a pace greater than the connective tendon, an imbalance occurs and knee pain will often result. OSD often occurs after the young person has experienced a growth spurt; OSD will almost always resolve itself through the normal growth processes of the body. While it is operable, OSD can cause pain in young athletes sufficient to limit their participation in competitive sports until the growth of the respective structures is even.
• Structural imbalances: When the musculoskeletal structure is not in balance or proportion, the movements required in sport will often create uneven forces to be directed into the joints, particularly the knee. Common imbalances of this type are differing leg lengths or a corresponding misalignment of the hip joints and pelvis structure. As the athletes propel themselves forward, as in all running sports, or from landing after a jump, the forces generated will be distributed unevenly, which creates both a misalignment of the knee at the point where forces are directed into the joint, and an inability to properly absorb or redirect the force. As all forces related to leg movement ultimately are directed through the knee joint, the consequences of imbalance are cumulative. This condition is most often genetic in origin; it will more often lead to wear and tear injuries as opposed to those occurring in a single incident.
• Structural predisposition: Female athletes have a structural predisposition to the serious knee injury known as a tear or rupture of the ACL. In some circumstances (most commonly when the ACL is damaged through a collision), the athlete will also experience a tear of the MCL. Numerous studies of this injury have determined that a female athlete may be five times as likely to sustain an ACL injury as a male athlete. This frequency results from the relationship of the knee in the female anatomy to the pelvis and the femur; the wider female pelvis, in proportion to femur length, causes a greater degree of force to be directed into the knee than typically occurs with males.

Other non-athletic conditions that may contribute to the impairment of knee joint function in sport are also wide-ranging. The most common are prior, non-athletic injuries, inadequate diet and nutritional practices, substance abuse, and conditions of either overweight or obesity, both previous and current.

Previous injuries unrelated to sport often create limitations for an athlete; these conditions may be unknown or not fully appreciated until the athlete undertakes training. Incidents such as a prior motor vehicle or industrial accidents may not have been treated in a comprehensive fashion at the time of the occurrence, leaving the true consequences, or sequelae, to be resolved, often incompletely, long after the fact. Growth plate fractures sustained by adolescents are an example; they are sometimes overlooked and dismissed as a childhood event; the fracture will often heal quickly, but improperly, creating a limitation of movement that is only discovered years later. The knee may have been the object of a prior partial ligament tear or prolonged bouts of tendonitis that become less manageable when training programs are undertaken; what is tolerable in ordinary daily living, such a small piece of cartilage floating in the joint, which causes occasional discomfort, may become impossible in a sport context where movements are made for maximum effect.

Diet and its companion, nutrition, are factors external to the structure of the body, though their influence is exerted relatively evenly on every aspect of joint health and development. Healthy bone and connective tissue growth and development require appropriate nutritional support; examples are ongoing supplies of minerals such as calcium, coupled with vitamins A and D, for the stimulation of strong, dense bones and teeth. Deficiencies in this supply to the body have a finite period within which they might be addressed; once a person reaches age 20, and sometimes sooner, the bones cannot be revisited in any biological fashion, and any weaknesses through poor dietary practices created during childhood and adolescence cannot be remedied.

The ideal weight of any athlete is never a finite figure; such weight will occupy a healthy range, subject to age, build, muscularity, the sports pursued by the athlete, and similar factors. At the ideal weight, the knee joint will be subjected to forces that it is likely able to tolerate. The further the athlete is from the ideal weight, the greater the risk that of an injury to the knee, as the joint will be bearing weight for which it is not equipped or designed. As the body stands, each knee bears approximately 50% of the weight of the body; as the body runs, the forces directed into each foot and up into each knee can approach three times the body weight of the athlete. Ten pounds (4.5 kg) of excess weight will translate into 30 lb (13.6 kg) of increased force with each stride; sports such as basketball generate similar forces on landing from a jump. An overweight athlete creates the dual risk of strain to the knee through wear and tear, as well as the creation of forces sufficient to tear or rupture a knee ligament.

see also Bone, ligaments, tendons; Genetics; Knee injuries.