As with any structure, the human body is built upon a framework that is constructed to carry out a wide range of functions. The bones, ligaments, and tendons are each essential parts of the human framework, integrated into a mechanism, the skeleton, that is crucial to the movement, stability, protection, and growth of the entire body.
Because bone is a hard, white, and seemingly sterile object, it is perhaps difficult to regard the human skeleton as an organism. The bones, ligaments, and tendons are not only a coordinated frame upon which human muscles and organs are attached and contained, the skeleton and the rest of the body are an interconnected biological device.
There are approximately 206 bones in the human body. Bones generally act as connective tissue within the body. All bones are composed primarily of the mineral compound, calcium phosphate, which is a very hard substance, with a lesser amount of collagen (a protein). For this reason, bone is structured as a relatively brittle matrix, although the softer collagen cells lend some degree of elasticity when a bone sustains trauma. Not all bones possess the same characteristics, nor do all bones fulfill the same purpose within the skeleton.
There are two types of bones: cortical bones and cancellous bones. Cortical bone has a denser, more compact structure than does the cancellous bone, and comprises the greater proportion of the skeleton mass. Most of the longer bones in the body are cortical, such as the femur (thigh bone). The long bones are hollow, and the center of the bone is filled with a substance known as yellow marrow. At the end of each long bone is an area known as the epiphysis, which is usually composed of cancellous bone cells; the marrow in this portion of the long bones is red marrow. Bone marrow is the body's manufacturing center for the production of erythrocytes (the red blood cells responsible for the transport of oxygen throughout the body), leukocytes (the white blood cells that neutralize and eliminate infection), and platelets (the organisms that cause blood to clot in the case of a laceration or internal bleeding). The red marrow generates the red blood cells required, and the yellow marrow produces the white blood cells.
Bone cells are further subdivide into three types: osteoblast, osteocyte, and osteoclast. Osteoblast cells are also known as bone-forming cells, which are located on or near the surface of the bone, producing osteoid, the compound used by the body in bone formation. Osteocyte cells are also known as bone-lining cells, which are essential to the maintenance of the bone structure. Osteoclast cells are usually inactive cells that will destroy other bone cell production.
Ligaments are the skeletal components that connect bones to other bones to create a joint, a flexible structure that is capable of movement and of bearing both weight and external forces applied to it. Ligaments are made of collagen, formed into short, fibrous bands. All of the body ligaments are important to efficient physical movement. However, the ligaments that are of special significance are the anterior cruciate ligament (ACL), the connector between the femur and the tibia (lower leg bone) essential to knee function, and the ulnar cruciate ligament (UCL), an elbow structure essential to the success of any baseball pitcher.
Tendons have a similar construction to that of ligaments: they are strong, fiber-like connectors. However, rather than connecting bone to bone, tendons connect muscle to bone, providing stability and a significant degree of resistance to external forces. Large tendons such as the Achilles tendon, which connects the gastroceus (calf muscle) and the calcaneus (heel bone), are able to withstand force due to the flexing qualities of the supple collagen that forms a large portion of the tendon structure. As with an ACL or UCL injury, a tear or rupture of the Achilles tendon is a debilitating occurrence.
The ACL is a commonly and severely injured ligament in the knee joint. Female athletes are far more prone to sustaining an ACL injury than males; National Collegiate Athletic Association (NCAA) research suggests that ratio of female basketball players to male players with an ACL injury is approximately 5 to 1.
This greater prevalence of this injury among female versus male athletes arises from the different shape of the female skeleton from that of the male. The relationship between the wider female pelvis and the shorter femur creates a more acute angle than that of the male anatomy. The line that one may draw between the hip and the knee is known as the "Q line." This more acute Q-line angle in female athletes results in greater forces being generated and directed in the knee joint. In a sport such as basketball, there are constant opportunities for the knee and the ACL to absorb the forces of explosive movements.
The skeleton of an athlete provides a number of indicators regarding prospective success in a particular sport. The size of the skeleton at certain points in the development of a child into adolescence, and later at the point where an adolescent nears full maturity, will provide an indication of the likelihood of competitive success in a particular sport. If a female athlete is 5 ft 3 in (1.6 m) at age 16, it is unlikely that she will grow significantly taller; basketball would present competitive challenges. However, skeletal size is a persuasive, but never a determinative, factor in athletic success. A lack of height, while a physical reality that limits an athlete in many sports, may be overcome by other equally important athletic considerations, such as intelligence, muscle strength, speed, reaction time, or explosive power.
The size of individual bones within the skeleton may also be a useful predictor of athletic ability. Tour de France cycling champion Lance Armstrong, regarded as a phenomenal climber, a rider able to master tough, mountainous routes, has a longer than normal femur (thigh bone), which gives him greater leverage as he delivers the pedal stroke to his bicycle. In a sport such as swimming, a narrow pelvis and broad shoulders will be an advantage in powering the body through the water, as the slimmer pelvis will generally create less drag.
The relative sizes of the torso (upper body) and the lower body are another indication of what sports may be most suited to a prospective athlete. The point in the body where the weight of the structure is said to be equal is referred to as the "center of gravity" or, alternatively, as the "center of mass." Considerations of the location of the center of gravity in sport are primarily those to do with stability of the athlete's physical movements. A low center of gravity will often translate into an ability to make coordinated movements that combine speed, balance, and power. A higher center of gravity will be useful to athletes who wish to combine speed with efficiency. The center of gravity is lower the longer the upper body. American football running backs and alpine skiers are examples of athletes who require a low center of gravity; successful marathon runners will possess longer legs relative to their torso, thus possessing a higher center of gravity.
Skeletal size and bone density are largely predetermined genetically. In addition to athletic injury or other trauma, there are a number of factors that will impact upon skeletal health. For example, osteoporosis is a disease of the bones that occurs due to low bone mass. Reduced mass makes the bone generally more fragile and susceptible to fracture. This condition is thought to cause or contribute to 1.5 million fractures in the United States annually. Osteoporosis can affect persons at any age; as bone mass and density peak at approximately age 30, the disease more typically affects older persons. Approximately two-thirds of osteoporosis cases occur in women, due to combined impacts of menopausal onset and a naturally smaller, slighter bone structure.
Athletic activities that require the body, and consequently bones, to bear weight will result in stronger bones, in that the natural loss of bone density will be slowed. The bones are a part of an organism that requires nutrients to remain viable. A deficiency in critical bone-maintaining minerals such as calcium (vitamin D) can create a weakened skeletal structure.
Studies of girls who participate in high school sports suggest that they absorb approximately 7% more bone-building mineral than sedentary girls, a factor likely to provide them with greater resistance to osteoporosis in their later adult years.