Introduction to Ergonomics

Oregon OSHA Course #201

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.

Cumulative Trauma Disorders

Cumulative trauma, or repetitive motion, disorders are disorders of the musculoskeletal and nervous systems which may be caused or aggravated by reptitive motions, forceful exertions, vibration, mechanical compression (hard and sharp edges), sustained or awkward postures, or by exposure to noise over extended periods of time.

CTDs can affect nearly all tissues - the nerves, tendons, tendon sheaths, and muscles, with the uppoer extrmeities being the most frequently affected. These painful and sometimes crippling injuries develop gradually over periods of weeks, months, and years, and result from repeated actions such as twisting and bending the hands, arms, and wrists. A common risk factor among these disorders is the use of force combined with repetitive motion over time.

The most common occupational diseases associated with CTDs are tendon disorders such as tendonitis, tenosynovitis, De Quervain's disease, trigger finger, Raynaud's syndrome, and carpal tunnel syndrome.

Tendon disorders are very common and often occur at or near the joints where the tendons rub against ligaments and bones. The most frequently noted symptoms of tendon disorders are a dull aching sensation over the tendon, discomfort with specific movements, and tenderness to touch. Recovery is usually slow and the condition may easily become chronic if the cause is not eliminated.

Tendonitis

Tendonitis is a form of tendon inflammatio that occurs when a muscle or tendon is repeatedly tensed from overuse of unaccustomed usage of the wrist and shoulder. With further exertion, some of the fibers that make up the tendon can actually fray or tear apart. The tendon becomes thickened, bumpy, and irregular in certain areas of the body (e.g., the shoulder), and the injured area may calcify. Without rest and sufficient time for the tissues to heal, the tendon may be permanently weakened. Tendonitis is common among power press operators, welders, painters, and assembly line workers (e.g., automobile, appliance, electronic production).

Tenosynovitis

Tenosynovitis is an inflammation or injury to the synovial sheath surrounding the tendon. These sheaths secrete synovial fluid which acts as a lubricant to reduce friction during movement. Repetitive motion using the hands and wrists may provoke an excessive secretion of synovial fluid, with the sheath becoming swollen and painful. It has been found that reptitions exceeding 1,500 to 2,000 per hour are known to produce symptoms associated with tendon sheath irritation in the hands. Tenosynovitis affects workers in jobs such as core making, poultry processing, and meatpacking.

Trigger Finger

Trigger finger, another tendon disorder, is attributed to the creation of a groove in the flexing tendon of the finger. If the tendon becomes locked in the sheath, attempts to move the finger will cause snapping and jerking movements. The palm side of the fingers is the usual site for trigger finger. This disorder is often associated with using tools that have handles with hard or sharp edges. Meatpackers, poultry workers, electronic assemblers, and carpenters are at risk of developing trigger finger.

Raynaud's Syndrome

Raynaud's syndrome, or white finger, occurs when the blood vessels of the hand are damaged as a result of repeated exposure to vibration for long periods of time. The skin and muscles are unable to get the necessary oxygen from the blood and eventually die. Common symptoms include intermitent numbness and tingling in the fingers; skin that turns pay, ashen and cold; and eventual loss of sensation and control lin the fingers and hands. This condition is also intensified when the hands are exposed to extremely cold temperatures. This illness is associated with the use of vibrating tools over time -- e.g., pneumatic hammers, electric chain saws, and gasoline powered tools. After long-term exposure -- perhaps 10 to 15 years working 6 to 7 hours a day with vibrating tools -- the blood vessels in the fingers may become permanently damaged. There is no medical remedy for white finger. If the fingers are fairly healthy, the condition may improve if exposure to vibration stops! or is reduced.

Other types of vibration may affect the entire body, producing overall fatigue and potential permanent damage. Vibration in conjunction with prolonged siting may also result in degenerative changes in the spine. For example, drivers of tractors, trucks, buses, construction machines, and other heavy equipment may suffer from low back pain, and permanent abdominal, spinal and bone damage.

Carpal Tunnel Syndrome

Carpal tunnel syndrome (CTS) is the compression and entrapment of the median nerve where it passes through the wrist into the hand -- in the carpal tunnel. The median nerve is the main nerve that extends down the arm to the hand and provides the sense of touch in the thumb, index finger, middle finger, and half of the fourth, or ring finger. When irritated, tendons housed inside the narrow carpal tunnel swell and press against the nearby median nerve. The pressure causes tingling, numbness, or severe pain in the wrist and hand -- often felt at night. Also, the pressure results in a lack of strength in the hand and an inability to make a fist, hold objects, or perform other manual tasks. If the pressure continues, it can damage the nerve, causing permanent loss of sensation and even partial paralysis. CTS develops in the hands and wrists from repetitive and/or forceful manual taks performed over a period of time. For example, the meatpacking injustry is considered one of the most hazardous industries in the United States because workers can make as many as 10,000 reptitive motions per day in assembly line processes, such as deboning meats, with no variation in motion. Consequently, stress and strain are placed on the wrists and hands, which can result in CTS.

Another CTD that accounts for a significant loss of productivity and large compensation costs to industry is back disorders. Next to the common cold and flu, a back disorder is the reason most often cited for job absenteeism. Pulled and strained muscles, ligaments, tendons, and disks are perhaps the most common back problems and may occur to almost half of the work force at least onece during a lifetime. Back disorders are included as an ergonomic hazard because the majority of workplace back disorders result from chronic, or long-term, injury to the back rather than from a specific incident. When back muscles or ligaments are injured from repetitive pulling and straining, the back muscles, disks, and ligaments can become scarred and weakened and lose their ability to support the back, making additional injuries more likely.

Back disorders are frequently caused by the cumulative effects of faulty body mechanics: excessive twisting, bending and reaching; carrying, moving, or lifting loads that are too heavy or too big; staying in one position for too long; poor physical condition; and poor posture.

Prolonged sitting stresses the body, particularly the lower back and the ghighs, and may cause the lower back (lumbar) region to bow outward if there is inadequate support. This abnormal curvature (called kyphosis) can lead to painful lower back problems, a common complaint among office workers.

Hearing Loss

Noise induced hearing loss is another disorder associated with repeated trauma. The degree to which hearing is affected depends on the intensity, frequency, duration of exposure, and individual susceptibility. Noise-induced hearing loss occurs gradually and can cause irreversible damage to the inner ear. Excessive noise in the workplace also is annoying and can affect worker performance. Research has shown that exposure to excessive noise also causes stress to other parts of the body resulting in increased muscle tension, a quickened pulse rate, and increased blood pressure. Workers exposed to noise sometimes experience nervousness, sleeplessness, and extreme fatigue.

Some industries where heavy machinery and equipment produce noise levels that may be potentially dangerous to worker health include sugar refining; iron and steel; boiler making; machine operations; textiles; forestry; mining; construction; agriculture; transport; automaking and shipbuilding.

Symptoms and Possible Ergonomic Cause

• Feet, legs ache, Standing in one place too long. varicose veins
• Lower back pain Trunk curved forward while standing or sitting.
• Shoulder and upper Arms outstretched, sideways, forward or upwards. arm pain Shoulders forced up due to position of work.
• Neck pain Head inclined too much backwards or forward.
• Forearm pain Unnatural grip. Static grip too forceful.
• Wrist pain Repetitive hand or finger motion.

What does the scientific literature tell us?

Low Back Disorders

The epidemiologic literature supports a relationship between the development of low back disorders and each of the following workplace risk factors: (1) lifting and forceful movements, (2) bending and twisting in awkward postures, and (3) whole-body vibration.

A NIOSH study in a grocery warehouse in Ohio illustrates the problem of low back disorders. Warehouse workers, performing long hours of repetitive, heavy manual lifting, had a rate of workers' compensation claims for back injuries of 16 per 100 workers, compared to the national average rate of workers' compensation claims for back injuries of between one and two cases per 100 full-time workers. In this warehouse, workers sometimes lifted a total of more than 3000 pounds in less than one hour. Clearly, this is a high risk workplace. As a matter of fact, workers compensation data from the National American Wholesale Grocers' Association and the International Foodservice Distributors Association for the years 1990 to 1992, found that back strains/sprains accounted for 30% of all injuries for warehouse workers. Data from the same report indicated that more than a third of all workers experience an annual injury in warehouse operations, accounting for a cost of $0.61 per worker-h! our. Many other workplaces like this one experience high rates of work-related musculoskeletal disorders.

Disorders of the Neck and Shoulders

For disorders of the neck and neck/shoulder region, the literature identifies two important workplace factors of (1) sustained postures causing static contractions of the neck and shoulder muscles (for example, working overhead in automobile assembly or in construction), and (2) combinations of highly repetitive and forceful work involving the arm and hand, which also affect the musculature of the shoulder and neck region.

Analysis of job components can show how workers develop musculoskeletal disorders. One study describes the job of a carbon setter, an important job in the processing of aluminum, which provides us with an example of repetitive and forceful work. Aluminum ore is melted at very high temperatures in large electric pots, about the size of a conference table. These pots develop a hard crust that must be broken in order to add materials to the melting aluminum. To break the crust, carbon setters frequently throw a large bar weighing twenty-five pounds into the pot. This report documented increased shoulder disorders among the carbon setters in the study.

The basic shoulder motion when throwing down the bar into the pot is the same motion involved in baseball pitching, (and we've all heard about million dollar rotator cuff injuries in the major leagues). In a typical game, a major league baseball pitcher throws between 90 and 120 pitches. A carbon setter throws the equivalent of between one and two double-headers every shift. The bar does not travel 90 mph like a baseball, but a baseball doesn't weigh 25 pounds.

Disorders of the Hand, Wrist and Elbow

There are several conditions to consider within the hand and wrist region. Combined work factors of forceful and repetitive use of the hands and wrists are associated with carpal tunnel syndrome. Vibration from hand tools like chainsaws (those that do not have vibration controlling mechanisms) also contributes to carpal tunnel syndrome.

Vibrating tool use has also been strongly linked to hand and arm vibration syndrome, a separate condition of the hand and wrist that affects the nerve and blood vessels.

Workers in industries such as meatpacking, garment work, fish and poultry processing spend their workdays performing forceful exertions and repetitive movements of the hand and wrist. The combination of these has been found to have a strong association with tendonitis of the wrist and have also been associated with disorders of the elbow, such as epicondylitis.

Length and Intensity of Exposure

The epidemiological literature indicates that the greater the level of exposure to a single risk factor or combination of factors, the greater the risk of having a work-related musculoskeletal disorder. The literature also indicates that an important factor is the time between each episode of exposure. With adequate time to recover or adapt, and particularly when lower forces are involved, there may be less harm to the body from repeated exposures. You will recall the carbon setter/major league baseball pitcher analogy; it is important to remember that a major league pitcher plays only every third or fourth game, while the carbon setter throws the equivalent of one or two double-headers five days a week, 50 weeks a year. The intensity as well as the extended length of the exposure to forceful, repetitive work plays a substantial role in the risk of work-related musculoskeletal disorder in many traditional occupational settings.

Psychological Factors

In workplaces with high rates of work-related musculoskeletal disorders there is little scientific evidence that the principal reason for the excess number of injuries or illnesses is the workers' psychological reaction to their workplace. However, there is evidence, particularly in office settings, suggesting that both physical and psychosocial [work organization] factors may be important contributors to musculoskeletal disorders. Work organization refers to the way work processes are structured and managed, and it deals with subjects such as scheduling of work, job design, interpersonal aspects of work, career concerns, management style, and organizational characteristics. We know more about how physical factors contribute to musculoskeletal disorders than we do about work organization factors. NIOSH is supporting research projects in both areas.

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