Bone Structure And Classification

In today’s blog we are looking at bone structure and classification, one of the many aspects of how the body moves that a personal trainer needs to understand to better train their clients.

Bone consists of a mixture of approximately 50% mineral salts, 25% water and 25% protein. Humans are born with cartilage, which over time becomes ossified (hard), turning into bone.

The bones in our body all come under one of five classifications:

1. Long bones
2. Short bones
3. Flat bones
4. Irregular bones
5. Sesamoid (seed-like) bones

Long bones – these are naturally longer than they are wide, for example the femur, the tibia or the phalanges in the finger. Long bones exist to create movement, especially in the arms and legs.

Short bones – these are typically as long as they are wide, and frequently a cuboid shape. Their shape offers a large surface area for muscles to attach to, so they are commonly found in quite complex joints, such as at the wrist and ankle, where a large range of motion is available.

Flat bone – this is a thin, cancellous bone, sandwiched between two compact layers. Its function is to protect major organs, for example, the cranium is a flat bone protecting the brain and the sternum is a flat bone protecting the heart and the lungs. The ribs are also classified as flat bones. Like the short bones, they provide a large surface area for muscles to attach to and pull from. Commonly, the muscles that move long bones are also connected to a flat bone. For example, the pectoralis major is connected to a flat bone as a base of support and inserted on to a long bone.

Irregular bones – the vertebrae are classified as irregular bones; there are 33 and each one is a slightly different shape.

Sesamoid bones – these bones develop in particular tendons at a site of friction or tension. Their function is to reduce wear and tear on the tendon, and also to increase the leverage of the muscle and its tendon over a joint. The most common sesamoid bone is the patella bone in the knee cap.

Focus on the long bone

As any personal trainer who has studied anatomy will know, a long bone is divided into different sections. Taking the example of the femur, at each end of this bone is a porous, spongy area known as the epiphysis; at one end there is also the epiphyseal growth plate, which is spongy or cancellous bone. It also houses much of the red bone marrow involved in red blood cell production.

In the middle shaft of the bone is a compact dense part of the bone known as the diaphysis, whose function is to transfer weight through the bone. It’s useful for a fitness instructor to understand this function, whether they are carrying out aerobic training or weight training. Running through the centre of the diaphysis is the medullary cavity, containing fatty yellow marrow. This is predominantly composed of adipose tissue and serves as an energy reserve.

The diaphysis and the epiphysis connect at the epiphyseal growth plate, which is one of the main sites for bone growth, through the lengthening of the diaphysis, which is promoted by the osteoblasts.

Finally there is the periosteum, a tough fibrous membrane, which wraps around the outside of the bone and gives it a supply of blood, nerves and bone-producing cells (osteoblasts and osteoclasts).

At the end of each bone is articulate cartilage, also known as hyaline cartilage, a hard white shiny tissue that gives the bone a smooth surface so that there’s no friction when it rubs against another bone. Osteoarthritis develops when this smooth surface becomes worn.