This material is for training purposes only. Its purpose is to inform Oregon employers of best practices in occupational safety and health and general Oregon OSHA compliance requirements. This material is not a substitute for any provision of the Oregon Safety Employment Act or any standards issued by Oregon OSHA. For more information on this online course and other OR-OSHA online training, visit the Online Course Catalog.

MODULE TWO: RISK FACTORS INHERENT IN THE WORKER

The worker brings certain ergonomic risk factors to the job.

It's important to understand that each worker's ability to respond to external factors, such as high force demands, is different and unique. The level, duration, and frequency of the loads imposed on tissues, as well as adequacy of recovery time, are critical components in whether increased tolerance (a training or conditioning effect) occurs, or whether reduced capacity occurs which can lead to cumulative trauma disorders CTDs as well as other musculoskeletal disorders (MSDs).

The Musculoskeletal System

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

The following personal risk factors may be generalized across populations, but it's important to understand that, when designing a workstation or procedure, you should design for the specific individual(s) working at that workstation. Among these are the following:

AGE

The prevalence of CTDs increases as people enter their working years. By the age of 35, most people have had their first episode of back pain. Once in their working years (ages 25 to 65), however, 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 insult.

Another problem is that advancing age and increasing number of years on the job are usually highly correlated. Although older workers have been found to have less strength than younger workers, 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. Other studies have reported a lack of increased risk associated with aging.

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 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.

PHYSICAL ACTIVITY

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, and after injury, the threshold for further injury is reduced.

We can define "fitness" for most physical activities as combinations 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 is generally accepted as a way of reducing work-related MSDs, the present epidemiologic literature does not give such a clear indication.

STRENGTH

Some epidemiologic support exists for the relationship between back injury and a mismatch of physical strength and job tasks. One important study found a sharp increase in back injury rates in subjects performing jobs requiring strength that was greater or equal to their isometric strength-test values. The risk was three times greater in the weaker subjects.

A second 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.

In another prospective study, it was found that reduced strength of back flexor muscles was a consistent predictor of recurrent or persistent back pain, but this association was not found for first time occurrence of back pain.

Strength vs. energy of work: Which is most important?

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.

There are four factors that increase forces compressing the L5/S1 vertebrae together:

Weight on the upper body
Deviation of the an erect posture stance
Weight of the load
Pull of the lower back muscles

ANTHROPOMETRY

Workers come in all shapes and sizes, and so designing for only the "average" person in one group or segment causes problems for everyone else who does not fit into that group.

Anthropometry studies the differences 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 squared), 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 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.

Some studies have reported that people with back pain, are, on the average, taller than those without it. A Finnish population study found that height was a significant predictor of herniated lumber disc in both sexes, but a moderately increased BMI was predictive only in men.

Severe obesity (exceeding 30 kg/m2) involved less risk than moderate obesity. One study found an association between obesity and radiological disc degeneration. However, another study of Finnish white collar and blue collar workers found no association between overweight, (relative weight >120%) and lumbosacral disorders either cross-sectionally or in a 10-year follow-up [Aro and Leino 1985].

Another study of workers in 11 factories found that short stature was significantly associated with pain in the neck and shoulder, but not in the back, forearm, hand and wrist. However in two other studies involving thousands of workers, height was not a factor for neck, shoulder or hand and wrist MSDs.

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.

So there you have it. Not everything you need to know, but it's a start. The only task left is the module quiz, so let's get to it.

MODULE Review Quiz

11. Which of the following is not described in the text as a critical component which can lead to cumulative trauma disorders CTDs as well as other musculoskeletal disorders?

a. level
b. duration
c. capacity
d. frequency

12. The musculoskeletal system is made up of all of the following parts, except:

a. Muscles
b. Lymphs
c. Tendons
d. Nerves

13. By the age _____, most people have had their first episode of back pain:

a. 25
b. 35
c. 45
d. 55

14. The age groups with the highest rates of compensable back pain and strains are the _____ age group for men, and _____ age group for women.

a. 30-34, 40-44
b. 29-34, 39-44
c. 25-29, 35-39
d. 20-24, 30-34

15. It is clear that differences seen with some MSDs is due to physiological differences rather than differences in exposure:

a. True
b. False

16. One important study concluded that the lack of workplace accommodation to the range of workers' height and reach may, in part, account for the apparent gender differences.

a. True
b. False

17. When physical fitness is examined as a risk factor for MSDs, results are mixed. One study reported that ______ of absenteeism could be explained by age, sex, and physical fitness.

a. 7%
b. 14%
c. 39%
d. 54%

18. When physical fitness is examined as a risk factor for MSDs, results are clear that it is a way of reducing work-related MSDs.

a. True
b. False

19. Demanding repetitive or static muscular work requires energy, not necessarily strength:

a. True
b. False

20. Anthropometric data are conflicting, but in general indicate that there is strong correlation between stature, body weight, body build and low back pain.

a. True
b. False

Congratulations! You've finished Module 2. In Module 3 we'll discuss the general ergonomics risks inherent in the task. See you there!

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