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Course 711 - Introduction to Ergonomics

Safety guides and audits to make your job as a safety professional easier

Risk Factors Inherent in the Worker

Factors inherent in the worker.
Click to enlarge.


It's important to understand that each worker's ability to respond to external demands of a task is different and unique. Workers are all individuals and they should not be lumped together into groups when considering ergonomic design. Stereotyping or making generalities about an employee's ability should not be based solely on any one of the factors such as age, gender, or strength. At the heart of ergonomic design is the idea that equipment should be designed so that it's able to meet the unique needs of each employee, not the general employee population.

Musculoskeletal system.

The Musculoskeletal System

The musculoskeletal system is made up of the soft tissue and bones in the body. Below are the basic parts of the musculoskeletal system.

  • Bones: the load-bearing structure of the body.
  • Muscles: tissues that contract to create movement.
  • Tendons: tissue that connects muscles to bones.
  • Ligaments: tissue that connects bones to bones.
  • Cartilage: tissue that provides cushioning and reduces friction between bones.
  • Nerves: the communication system that links muscles, tendons, and other tissue with the brain.
  • Blood vessels: tubes that circulate nutrients throughout the body.

Factors Inherent in the Worker - Age


The prevalence of CTDs increases as people enter their working years (ages 25 to 65). By the age of 35, most people have had their first episode of back pain. Once in their working years, the prevalence is relatively consistent. Musculoskeletal impairments are among the most prevalent and symptomatic health problems of middle and old age.

Nonetheless, age groups with the highest rates of compensable back pain and strains are the 20-24 age group for men and 30-34 age group for women. In addition to decreases in musculoskeletal function due to the development of age-related degenerative disorders, loss of tissue strength with age may increase the probability or severity of soft tissue damage from a given injury.

Another problem is that advancing age and increasing number of years on the job are usually highly correlated. Older workers have been found to have less strength than younger workers, although hand strength does not appear to decline with aging. In one study, average hand pinch and grip scores remained relatively stable in their population with a range of 29 to 59 years.

Factors Inherent in the Worker - Gender


Whether the gender difference seen with some MSDs in some studies is due to physiological differences or differences in exposure is unclear. One 1991 study found no gender difference in workers compensation claims for Carpal Tunnel Syndrome (CTS). Another study found no gender difference in reporting of neck or upper extremity MSD symptoms among newspaper employees using video display terminals (VDTs).

In contrast, other studies have reported that neck and shoulder muscular pain is more common among females than males, both in the general population and among industrial workers.

An important study noted that significant gender differences in work posture were related to stature and concluded that the lack of workplace accommodation to the range of workers' height and reach may, in part, account for the apparent gender differences. Also, the fact that more women are employed in hand-intensive jobs and industries may account for the greater number of reported work-related MSDs among women. Another study reported that men were more likely to develop DeQuervain's disease* than women; they attributed this to more frequent use of hand tools.

*DeQuervain's Disease is an irritation and swelling of the sheath or tunnel that surrounds the thumb tendons as they pass from the wrist to the thumb.

Factors Inherent in the Worker - Strength and Endurance


Muscular endurance is the ability of a muscle or a muscle group to remain contracted over a period of time. There are two types of endurance: static and dynamic.

  • Static endurance can be determined by the length of time a limb can maintain a certain position.
  • Dynamic endurance can be measured by the number of times a limb can perform a movement against a certain resistance.

Muscular strength is the maximum amount of force that a muscle can exert under maximum contraction. The amount of force that can be exerted by the muscles in the arms, legs and back depends on body posture and the direction of force. For example, when standing, you can exert more force when pulling backwards than when pushing forwards.

A study evaluated the risk of back injuries and strength and found the risk to be three times greater in the weaker subjects. They found that job matching based on strength criteria appeared to be beneficial.

Factors Inherent in the Worker - Physical Fitness


The relationship of physical activity and MSDs is more complicated than just "cause and effect." Physical activity may cause injury. However, the lack of physical activity may increase susceptibility to injury. A lack of physical activity after injury may increase the risk of further injury.

Fitness for most physical activities is a combination of strength, endurance, flexibility, musculoskeletal timing, and coordination. In a study of male fire fighters, physical fitness and conditioning appeared to have significant preventive effects on back injuries. However, the most fit group had the most severe back injuries.

When physical fitness is examined as a risk factor for MSDs, results are mixed. One study reported that only 7% of absenteeism could be explained by age, sex, and physical fitness. On the other hand, another study found that physical capacity was related to musculoskeletal fitness.

Although physical fitness and activity are generally accepted as ways of reducing work-related MSDs, the present epidemiologic literature does not give such a clear indication. However, there is clear evidence that stretching exercises do have a positive effect on the reduction of MSDs.

Check out this short audio clip by Dan Clark of the that gives a good introduction about the importance taking stretch breaks during work.


Repetition and static work requires energy.

Strength is important, but not necessarily the key. "Heavy work" stresses the heart and lungs which may result in rapid fatigue - general or localized. The probability of injury increases as muscles weaken. Consequently, demanding repetitive or static muscular work requires energy, not necessarily strength. You may be strong, but not have sufficient energy to do the task.

  • Isometric (Static) work occurs when the muscles remain contracted for an extended period of time with no movement, as in holding an object. Muscles do not get the chance to relax and can fatigue quickly.
  • Isotonic (Dynamic) work involves repetitive contraction and relaxation of the muscles and is less tiring and more efficient than static work. The motion of the muscles also help pump blood supplying oxygen and eliminating carbon dioxide.

The image to the right illustrates static and dynamic work. The man is performing pushups. His legs are performing static work and his arms are performing dynamic work.

Factors Inherent in Workers - Anthropometry

Make work stations adjustable to fit different workers.
Click to enlarge.

Anthropometry is the science of studying the difference in body size and proportions by measuring various body characteristics, including weight, physical range of mobility, and body dimensions. This information is then used by designers to engineer tools, equipment, furniture and workstations for maximum efficiency for each individual worker.

Weight, height, body mass index (BMI) (a ratio of weight to height), and obesity have all been identified in studies as potential risk factors for certain MSDs, especially Carpal Tunnel Syndrome (CTS) and lumbar disc herniation.

The relationship of CTS and Body Mass Index (BMI) has been suggested to relate to increased fatty tissue within the carpal canal or to increased hydrostatic pressure throughout the carpal canal in obese persons compared with slender persons. Carpal tunnel canal size and wrist size has been suggested as a risk factor for CTS, however, some studies have linked both small and large canal areas to CTS.

Anthropometric data are conflicting, but in general indicate that there is no strong correlation between stature, body weight, body build and low back pain. Obesity seems to play a small but significant role in the occurrence of CTS.

Ergonomic Risk Factors

Here's a great NAPO video entitled "Lighten the Load." Hope you find it entertaining!

Module Quiz

Before beginning this quiz, we highly recommend you review the module material. This quiz is designed to allow you to self-check your comprehension of the module content, but only focuses on key concepts and ideas.

Read each question carefully. Select the best answer, even if more than one answer seems possible. When done, click on the "Get Quiz Answers" button. If you do not answer all the questions, you will receive an error message.

Good luck!

1. Each worker's ability to respond to external demands of a task is _____.

2. The musculoskeletal system is made up of all of the following parts, EXCEPT _____.

3. What are the two types of muscular endurance?

4. Which type of "work" is less tiring and more efficient?

5. The study of the difference between body size and proportions by measuring various body characteristics is called _____.

Have a great day!

Important! You will receive an "error" message unless all questions are answered.