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:
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
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.