Well, actually this is about bones - but blood and guts got your attention, didn't it?
Last summer, during the slow months, we ran some articles on the mechanics of jumping and spinning. This summer we are going to treat you to a tour of the human body and how it works in relation to sports. This should keep us amused, at least until the season starts up again in August.
When analyzing the performance of the human body in a sport, the body can be thought of as a self propelled machine. The skeletal system is the framework of the machine, the muscular system is the motor, and the nervous system is the control system. Food is the fuel, and its digestion and metabolism in the digestive system and muscles is the combustion process for the motor.
The study of the motion of an object (be it a car or an athlete) without worrying about the forces that control its motion is called kinetics. When the forces that determine the motion are included, the resulting field is called mechanics. The science of mechanics applied to living things (primarily muscles) is called biomechanics, and the study of the kinetics of humans motion is called kinesiology. In sports that have adopted a modern approach, these fields have become an integral part of their development. In skating, whose training methods in most countries, including the US, are caught in a 19th century time warp, the application of these fields has been limited.
The human skeleton is made up of 206 bones. The places where bones are joined together are called joints. There are three types of joints, fibrous, cartilaginous, and synovial. In fibrous joints the bones are connected by short fibrous tissue, or by longer fibrous tissue called ligaments. In cartilaginous joints the bones are connected by cartilage, a smooth elastic translucent tissue. Fibrous and cartilaginous joints are either immovable, or only slightly movable. In later years some of these joints completely ossify (are converted to solid bone material). The synovial joints are the most freely movable joints. In synovial joints the bones are separated by a fluid containing joint cavity. The opposing bone surfaces in the joint cavities are covered with a layer of cartilage. Outside the joint cavities the bones are connected by ligaments. In addition to being held together by ligaments, synovial joints are also stabilized by the muscles around the joints.
Bones are made up of material rich in calcium minerals and living bone cells. Bones are porous and contain within them soft connective tissue called bone marrow. The circulatory system extends into bones, directly supplying the bone cells and bone marrow. Blood cells are formed within bone marrow. Cartilage does not have a direct blood supply, and instead receives its nutrients from the fluid that surrounds it, making it very slow to recover from injury. Because the circulatory system extends into bones, the blood supply to regions of a bone can be cut off if the blood vessels are also damaged in the course of a severe bone break. If this occurs, the region of bone cut off from the blood supply dies and must be removed. This is the type of injury that ended the football career of Bo Jackson, who required hip replacement surgery as a result of a severe break of a femur.
When fetal bones first begin to form they are initially made of cartilage. At six weeks bone tissue begins to develop, but even at birth bones are still a combination of bone and cartilaginous tissue. During childhood and adolescence there are regions in each bone where cartilaginous tissue is formed and then converted into bony material. These regions are called growth plates. In long bones, such as the bones of the arms and legs, there is a growth plate near each end of the bones. At birth, joints are fairly open, and underdeveloped, allowing a range of motion exceeding that of fully developed joints. By age 6 joints are fairly well developed. Some experts in dance recommend that children below this age not train excessively until the joints in their feet and hips are properly developed.
There are two types of bone cells. The function of one type is to build up (or repair) the solid material of the bone, the function of the other is to remove (dissolve) it. In a growing child the first type is more active than the second, but even during the growth years the second type is active, helping to shape the bones as they grow. At a certain age (mid to late teens) the growth plates become inactive, the soft material there is totally converted to bony material, and the growth plate is said to have fused. This phase of development can be identified by an x-ray.
Once the growth plates fuse, bones no longer increase in size. Nevertheless, both types of bone cells continue to work, but now at a balanced rate, one group of cells building up material, the other dissolving it away. On the average the solid material in a bone is replaced every 7 years. In later years the cells that build up bony material become less active and bone mass decreases. This is one cause of the reduced height of a person as they pass middle age. This effect is more pronounced in women than in men, and also appears to be affected by rigorous long term athletic training. For example, it is known that the bones of women track athletes deteriorate at a much faster rate than for the general female population. The same is likely to be true for women skaters, but few studies have been done on that subject.
Bones do more than provide the framework for our body. They provide protection for internal organs, working with muscles they allow movement (such as skating), they are a storage location for fat and various minerals used by the body, and some of them are the main locations within which blood cell formation takes place. The structure of bones allows them to be remarkable strong and durable without being brittle. Bones can resist 25,000 lb/in2 of compression and 15,000 lb/in2 of tension. Nevertheless bones are susceptible to traumatic injury in the form of bruises and fractures, and overuse injuries such as growth plate separations and stress fractures
In recent years, as younger athletes have begun to train more strenuously, growth plate separations have become more common. It is important that a growth plate separation be properly treated, so that the growth plate does not prematurely fuse, preventing full bone growth. Stress fractures result from repetitively stressing a material in one region (flexing or bending it, for example) until it develops small cracks. Carried to the extreme, the material will break. [Stress fractures in metal are what allow you to break a cloths hanger by bending it repeatedly.] Stress fractures in the leg and foot bones of skaters are not uncommon.
Another condition that can affects the skeletons (specifically the knees) of growing children is Osgood-Schlatter disease. It tends to occur in girls between ages 8 and 13, and in boys between ages 10 and 15. It is three times more common in boys than in girls. In this condition the patella ligament is stressed to the point it begins to tear away from the tibia by fracturing the bone in the area where it attaches to the tibia. It is aggravated by kneeling, running, and jumping. The usual treatment is rest, and to lessen the stress on the ligament until it is firmly attached to the tibia. This may require 6 months to a year off the ice. Imbalances in the muscle groups of the upper leg may also be a contributing factor to its occurrence, as they are to other injuries to the knees (by reducing the stability of the knees).
The large range of bodily motion required for athletic activity originates in the synovial joints. More on these next time.
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