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Concepts in Human Factors Engineering (Part 2 of 11)

Concepts in Human Factors Engineering (2): The Fragile Skeletal System

by Dennis R Andrews PhD, PSP, CECD

By Dennis R Andrews PhD, PSP, CECD (ExpertPages member profile page)

The musculoskeletal system consists of the muscles and bones of the human body. The muscles act as the force while the bones act as lever arms. Bones are articulated and connected by ligaments and tendons. Ligaments can be considered as the point of articulation while tendons the link to muscular energy. When we talk about articulation we refer to moment, lever arms or torque, which is supplied by ligaments and the force by the muscle. The skeletal system is the framework of the body and without a strong frame the body would simply collapse upon itself. Humans are different in many aspects and consequently the probability of injury is more of an individual value than the general population value. The spine consists of 7 cervical vertebrae, 12 thoracic, 5 lumbar and the sacrum Coccyx. The skeleton is subjected to various forces and injury mechanisms during daily activities such as “tension, compression, torsion and shear.” Injuries occur when the forces and mechanisms surpass the threshold of the specific bone or muscle. The mechanisms can be as described above or in combination such as compression and torsion simultaneously.

Concepts in Human Factors Engineering is a series containing eleven articles:
  1. An Overview of Anthropomorphic Data Gathering
  2. The Fragile Skeletal System
  3. The Energy Force of Our Frame
  4. The Message Delivery System
  5. The Body as a Machine
  6. The Oxygen Machine
  7. The Body’s Transportation System
  8. Human Body Energy
  9. A Hot Workplace
  10. The Rhythm of Working
  11. The Bionic Worker of the Future
Some bones are stronger than others based upon differences in mineral content, collagen fibers, age, and pre-existing weaknesses. Long bones such as femur, radius, and humerus consist of a shaft and ends, which broaden into an articulated joint. If this joint becomes weak or develops excessive friction the probability of injury increases and pain develops. Stress on the musculoskeletal system increase the potential for injury and pain as age advances. Women have a higher probability and a lower threshold of injury than men, probably because women generally have weaker muscles.2 Flat and long bones consist of compact and spongy materials; compact material is denser than spongy material. Cortical or compact material make up the long shaft while cancellous material is found at the ends of the bones. This probably accounts for bones being dislocated with less force than broken. While bones may be considered having the properties of plastic deformation bones do have some restitution (elastic properties) and can resist some permanent deformation. Bones of older persons, usually over 30 years of age, become more plastic and less resistance to deformation. The strength of the bones comes from the size of the cross-section area, which would increase the breaking threshold. Geometry plays an important part in analyzing bone strength using the standard formula for area.

Cartilage is an important component of the musculoskeletal system especially in the knee area. Cartilage is divided into three types and is firm but elastic, a flexible substance with amazing stretching properties. Cartilage is smooth when found at the articulated areas of long bones, which helps in reducing friction and damage to joints. The disks of the vertebrae are considered fibrocartilage the knee has hyaline cartilage and the nasal area has elastic cartilage. Connective tissues are ligaments when they connect bones and are tendons when they connect muscles as well as fascia when they cover organs or muscles. Fascia tissue are wrapped in sheaths and coupled with viscous fluid, synovia, reduces friction and thereby prolongs painless articulation during one's lifetime. Articulation involves a single degree of freedom or multiple degrees of freedom. Consider a door hinge as an articulated joint this would be a single degree of freedom but the knee joint, or wrist would be considered multiple degrees of freedom. Spinal disk injury usually occurs when force squeezes fluid from the disk creating a herniation. A herniation may be difficult to interpret by professionals by it can be seen with appropriate x-rays. The location and direction of the disk herniation can help to determine the direction of force applied at that section of the spine. The spine has cartilaginous joints in which the disk is positioned between two vertebrates. When disks of the vertebrae are misaligned spinal nerves may be pinched creating a sensation of pain. Several spinal conditions can create an increased potential for injury such as scoliosis, lordosis and kyphosis. Scoliosis is a condition of the spine, which is curved laterally, kyphosis is consistent with a hunchback condition and lordosis is a condition where the natural curve of the spine is more pronounced.3 The amount of load on the spine is a product of the condition of the spine as well as its curvature. A straight spine with no curve would have a higher compression loading then a spine with a natural curve. The spine is highly susceptible to injury in motor vehicle accidents given the effects of ramping (compression), torque (spinning vehicle) and shearing (cervical displacement in relation to torso).4

Range of motion is determined by friction between joints, pre-existing injuries, and to some degree athleticism. Range of motion for a population in general is not the same since some people are more flexible than others and as age advances the flexibility decreases. It is generally agreed that women have more flexibility than men. When joints are no longer functioning properly or produce constant pain artificial joints are usually considered. Many athletes undergo knee surgery and over the years artificial knee joints are used. One of the main joint replacements with an artificial joint for the elderly are the hip joint, although the hip joint replacement does not always provide relief from limited movement or pain. The artificial devices must not be rejected by the body and must be strong and nontoxic. A repetitious task will have excessive wear on the joint that is most involved with the task. With this in mind periodic breaks should be taken and different movements should be incorporated whenever possible.

In summary to the skeletal system with its many components is highly complex. Some people attempt to equate the skeletal system with a mechanical system including forces and levers. This may be appropriate for a general analysis by an in-depth analysis requires much more. Movement of the skeletal limbs requires activation of muscles using ligaments and cartilage. Nerves transmit sensory perception, which enhances the accuracy of the movement of the muscles and skeletal mechanisms. The spinal column is probably the most sensitive and fragile system in the human body. In illustrating its fragility a person can injure their spine, by simply bending down and picking up an object. On the other hand in some severe motor vehicle impacts the spine will not be injured. The spine continues to be the subject of much research and analysis, even to the point of using human subjects. Human subjects are a poor selection for determining spinal injuries since the code of federal regulations prohibits the probability of injury to that of a physical examination.

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