The Skeletal System

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  • File photo | Credit Getty Images: ROGER HARRIS, Science Photo Library

Different Functions of the Skeletal System

The skeleton is made up of about 206 bones (in adult) which allow the human body to withstand its own weight with little or no muscular effort involved to stay upright and aligned (Figure 2.7). Apart from this, the most important functions of the skeletal system include:

  • To serve as a rigid structure of mechanical stability, to support soft tissues and serve as attachment points for muscles
  • To protect vital organs (brain, heart, lungs, spinal cord) and nerves
  • To break down and regenerate bone (bone cells continually do this)
  • To produce blood cells (in the red bone marrow)
  • To assist in movement (skeletal muscles move bones) by making force and torque transfer efficient
  • To store minerals (particularly Calcium (Ca) and Phosphorus (P))
  • To store chemical energy (triglycerides, in the yellow bone marrow)
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In a locomotive sense, the skeleton consists of a number of specialized bones suited for different purposes and loading profiles. The way the skeleton is designed, with an upright spinal column and long extremities with different bone widths and sizes, is the result of evolutionary requirements for human survival and development, in terms of structural strength, mobility, and flexibility. For example, the lower extremities (the legs) are quite wide and strong, and evidently suited for strength and stability in the lengthwise direction of the long bones (the femur over the knee, the fibula and tibia under the knee), greatly enabling us to stand, walk and run. Conversely, the arms (the upper extremities) consist of smaller, more complex bones that are developed to have maximum mobility and high precision, but (comparatively) low strength. This is because human survival has been highly dependent on our ability to move quickly and endure a lot of standing and walking, but also to use our hands as high precision sensors and tools, causing a development of intricately attached small bones.

Some bones do not have the long shape nad form of those in the extremities; some appear to be more like small, tightly clustered bones that are connected tightly and often form a base for complex-functioning body parts, particularly the bases of the feet and hands. In the hands, these bones are known as the carpals (carpus is Latin for wrist) and they form protective armour around a number of blood vessels, nerves and important tendons that allow finger movement. These all pass through a narrow passage in the wrist known as the carpal tunnel. The corresponding clusters of bones at the base of our feet are called the tarsals (Latin for ankle). On both the hands and the feet, the bones that extend out to our fingers and toes are known as phlanges.

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Since it contains blood, bone cells, energy and minerals, bone is a living material with a capacity to adapt itself to the type of loading it is under, and the body is continually breaking down or regenerating bone. It is essential to load the skeleton in order for bones to grow (this stimulates increased development of collagen fibres and more deposition of minerals, making the bones thicker and stronger) — if the bones are not placed under any type of stress, the body’s processes of breaking down and reabsorbing bone materials overtakes the bone generation and the bones brittle and weak — a well-known phenomenon among old people, people who are bedridden long periods of time and astronauts due to weightlessness. This condition of bone fragility, where bone resorption processes outpace new bone development and mineral deposition, is known as osteoporosis. Such bones become so brittle and weak that a very small force application may break them, for example resulting in a hip fracture just from sitting down too quickly.

Joints

Joints are the structures that appear at the points of contact linking bones to other bones, to cartilage or to teeth.

Some joints are simply links between two bones without permitting movement at all, while others are specifically designed to permit movement, or at least a bit of flexibility. Joints that allow movements in one dimension (translation, or “gliding” movement) may for example be found between the smaller bones in the wrist or where the ankle meets the foot. Two-dimensional joints, in many cases also known as hinge joints, allow rotation of bones relative to each other and are found, for example, in the elbows, knuckles and knees. Finally, three-dimensional joints permit the greatest range of movement in several dimensions, and are for example found at the base of the thumb (a so-called saddle joint) or at the shoulder and hip joints (ball-and-socket joints).

The type of joint that permits movement in one, two or three degrees of freedom is called a synovial joint. Such joints are always between articulated bones (bones that meet and form a joint) whose ends are covered by a bendy, tough layer of articulate cartilage, which reduces friction when the bone ends move (translate or rotate) relative to each other.

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Synovial joints always have a synovial cavity in which the bone ends move against each other, and are surrounded by a capsule filled with joint fluid that lubricates the articular cartilage and allows even smoother movements with less friction between the bone ends. The capsule is covered by an outer layer of dense, tough connective tissue that is flexible enough to permit movement, but strong enough to keep the bones from dislocating. Depending on which joint it is, there may also be a presence of ligaments, which are bundles of fibrous connective tissue that are especially designed to withstand high strains.

Due to their complexity and the presence of many complicated and fragile structures passing through them, joints are particularly sensitive to injuries caused by physical loading in extreme positions.

The cartilage at the end of articulated bones is thickest in the middle, meaning that working in extreme joint angles may result in wear and tear on the thinnest part of the protective cartilage layer. As we age, the risk for joint problems and injuries increases due to a number of factors:

  • Decrease in the production of synovial fluid, reducing both the lubrication of the joint cartilage and the transportation of unwanted substances away from the joint
  • Articular cartilage between the bones becomes thinner
  • Individual genetic and lifestyle factors
  • Life-long wear and tear on the joint
  • The fibrous ligaments around the joint capsules lose flexibility and become shorter, reducing the protection against movement-related injury and bone dislocation

Injuries and Healing

When it comes to withstanding physical loading, the body is protectively structured in such a way that:

  • The skeletal muscles protect the skeleton
  • The skeleton protects the inner organs
  • The joints protect blood vessels, tendons, muscles and nerves that run through them, but are also the most fragile structure of the three.

When any of these three structures are subjected to increasing mechanical forces, it is said that they are placed under strain until they can no longer withstand the force, and then they break. This stage is called trauma, and it means that the structures are injured and need time to heal before they can perform normally and take on more physical loading.

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The muscle tissues are soft and can cushion the body (up to a point) from applied forces. The supply of plenty of blood flow, allowing the transport of nutrients and removal of unwanted materials, means the healing of mild to moderate muscle injuries typically takes place in a matter of weeks – more severe strains can take months.

The skeletal bones are excellently suited to withstand long-term loading in numerous directions (Figure 2.11) and static loads (such as the weight of our own body when we stand up), but because they have less blood flow and the required mineral deposits to create new bone take a long time to deposit, bones take longer to heal when injured.

Injuries (breaks) in bone structures are called fractures, and usually heal in a matter of 5 to 6 weeks. Additionally, the healing times vary considerably depending on a number of factors, such as the injured person’s age and general health, the site and severity of the fracture, the type of bone that has been fractured, proximity to a joint, infections, etc.

Joints are the most complicated and fragile structures of the three, partly because they consist of many different kinds of tissues and structures, but also because they are supplied with the least amount of blood flow (particularly ligaments). For this reason, injuries to joints can take months to years to heal, and depending on the age at which the injury is sustained, damaes may be permanent. So, the priority order for work design is to avoid unnecessary risk of injuries first to joints, then the skeleton, then the muscles.

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  • References
    • Dray, S.M. (1985). Macroergonomics in Organizations: An Introduction. In. Brown, I.D., Goldsmith, R., Coombs, K. and Sinclair, M. (Eds.) Ergonomics International, 85: 520 – 525. Taylor and Francis, London.
    • Hendrick, H. & Kleiner, B. (2001). Macroergonomics: An Introduction to Work System Design. Santa Monica, CA: Human Factors and Ergonomics Society, Design, Human Factors & Ergonomics Society. ISBN 0-945289-14-6
    • Human Factors and Ergonomics Society. (2015). Technical Groups.
    • IEA, International Ergonomics Association. (2000). Definition and Domains of ergonomics.

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