Structure of bone
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Bones are classified as being either spongy or compact. Spongy bone has a honeycomb appearance and is found at the ends of bones. It is usually covered by hyaline cartilage, which is a tough, durable substance that helps prevent wear and tear. Compact bone runs the length of the bone and consists of a central canal surrounded by concentric ring-shaped calcium plates. The whole of the length of a long bone is covered by a tough connective tissue called periosteum. This connects muscle to bone via tendons. The bone contains calcium which makes it harder and collagen which helps to make the bone both light and strong. The shaft of a long bone is known as the diaphysis, and the ends are known as epiphyses.
There are five different types of bone, which are classified by their shape rather than their size:
- Long bones, such as the tibia, fibula, ulna, radius, phalanges, humerus and femur. These bones are not solid, but contain bone marrow in the centre. They are strong, but not too heavy.
- Short bones, such as the carpals, metacarpals, tarsals, metatarsals and calcaneous (heel bone) in the foot. These are usually short and square in shape. They are spongy with an outer layer of compact bone, and are light and strong.
- Irregular bones, such as the face bones and vertebrae. They are spongy on the inside, and are surrounded by an outer layer of compact bone.
- Flat bones, such as the cranium, ribs, pelvis and scapula. These bones have a large surface area which helps to protect vital organs and allows the attachment of a variety of muscles.
- Sesamoid bones, such as the kneecap (patella). These bones help to ease joint movements and resist friction and compression.
Bone is formed by the process known as ossification. Ossification refers either to the process of bone formation or to the conversion of fibrous tissue or cartilage into bone. Some bones, including the flat bones of the skull, form directly in membranes. This process is known as direct ossification. The short and long bones are formed by the gradual replacement of hyaline cartilage, from the foetal stage of development up until full maturation in the late teens. This process is known as indirect ossification.
A disc of bone separates the bone in the diaphysis from the bone in the epiphysis, and it is called the growth plate (epiphyseal disc). As the child grows, it is only at this place where an increase in the length of the bone can take place. On the outside of the bone, periosteal ossification continues. Osteoblasts (bone forming cells are laid down at this plate, while osteoclasts (bone destroying cells) remodel the bone to maintain its shape.
Damage or injury to the epiphyseal plate of the growing bone may result in abnormalities in growth. If the entire width of the plate is affected, bone growth in terms of length may be retarded, or will stop. If part of the plate is affected, bone growth will be unequal across the plate, resulting in abnormal shape or deformity. It is, therefore, essential that when training with children, considerable care should be taken. Moderate intensity will have a positive effect on bone development, whereas heavy or strenuous work could lead to ‘overuse’ injuries or damage to the epiphyseal plates.
There are many beneficial effects for the skeletal system through exercise:
- Bone has the ability to alter its strength in response to mechanical stress. By the process of bone formation and resorption, there is increased deposition of minerals and increased formation of collagen fibres. Both contribute to increased bone density and strength. The deposits of minerals and collagen fibres align themselves along the lines of stress produced by the external mechanical force, such as the force of gravity or the pull of muscles. Therefore, the bone is strengthened to withstand specific mechanical forces. In addition, there is improved production of bone marrow.
- Some people subject their bones to high levels of repetitive mechanical stress. Their bones respond by becoming thicker and stronger. However, if the stress and repetition is excessive, bone resorption may exceed formation. In specific sites, such as the foot and lower leg, stress fractures (painful weakened areas of bone) may occur. These are common in endurance athletes, who train regularly over long distances.
- Hyaline cartilage thickens which helps to cushion the joint, therefore protecting the bones from wear and tear.
- Tendons become thicker and become able to withstand larger forces.
- Flexibility and mobility training may slightly stretch ligaments, allowing a greater range of movement at the joint. Although too great a stretch of the ligaments can lead to joint instability.
- In the absence of mechanical stress, as occurs during periods of inactivity, immobility or bed-rest, bone absorption outstrips formation. This results in weakened bone. This process starts after the age of 30 in females, and accelerates rapidly after the age of 45 as oestrogen (female sex hormone) levels drop, until up to 30% of calcium is lost by 70 years of age. Generally, this process starts in males after the age of 60. Furthermore, there is a loss of collagen with ageing, which causes the bone to become more brittle and lose its tensile strength. This is why osteoporosis (brittle bone disease) and fractures are more common in the elderly. Weight-bearing exercise (where the spine is loaded) can help prevent or alleviate osteoporosis.