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Course 711 - Introduction to Ergonomics

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Ergonomics - Back Problems

Back pain is one of the more common computer user complaints. The back is a very complex structure; back problems can result from several causes.

Problem: A chair that fails to support the lumbar (lower) region of the spine is a common cause of back discomfort, because up to 35 percent more pressure can be placed on the lower back when sitting. The normal alignment of the spine, if viewed from the side, is an S-shaped curve with an inward curve at the neck, an outward curve in the middle of the back, and an inward curve at the lower back, the lumbar region. When a chair does not provide adequate lumbar support, the lower curve of the back flattens (this is called lumbar lordosis). When a person is seating himself, the bottom of the hipbone contacts the chair first. As the sitting process is completed, the hip actually rotates backward, flattening the curve in the lower part of the back. This causes the spinal discs to stretch from the vertebrae, causing back discomfort.

Solution: A chair that provides good low-back support and has the backrest set at the proper height can maintain the normal alignment of the lower spine, relieving fatigue and discomfort.

Problem: A straight-back chair provides little or no support to the lower and upper back. Sitting in this type of chair causes back muscle fatigue as a result of muscular efforts to maintain back posture for a long time.

Solution: A tiltable backrest allows the user to change positions, reducing muscular effort and fatigue from sitting. A slight backward recline also helps to reduce the flattening of the lower spine when sitting.

Problem: When a chair is too soft, the user sinks into the seat pan. This restricts movement and causes thigh, buttock, and lower back fatigue. Conversely, when a chair is too hard, a user may need to change postures frequently to relieve thigh and buttock discomfort.

Solution: People spend much of their time at work sitting. This is especially true of computer operators. A computer users chair should be designed to allow free movement while sitting. The chair must be properly designed for comfort, efficiency, and the task. Because computer users chairs are very personal items, users must be involved in the selection and purchase of chairs. This will ensure that users are satisfied with their chairs and that the best chair has been selected for each user.

Problem: When a display screen is too low, it causes the operator to lean forward, slouch down, or lower his or her chair to improve screen viewing. This can cause the lower curve of the back to flatten as a result of no lumbar support.

Solution: Raise the monitor to the correct viewing height, so that the topmost active line of the character display on the screen is at or just below the operators eye level.

Neck problems

Neck strain is also a common complaint, and causes are often related to the computer monitor height, the absence of a document holder, or improper positioning of the holder.

Problem: The monitor is too high or low, causing the user to bend the neck backward or forward to see the screen.

Solution: Lower or raise the monitor to the correct viewing height, as recommended under "Display screens" on page 6.

Problem: Documents placed flat and off to the side of the work surface cause forward bending and twisting of the neck.

Solution: An articulated document holder or a document holder mounted on the monitor, positioned at the same elevation as the monitor screen, should relieve this problem. A document holder should be usable on either side of the monitor, between the keyboard and the computer screen.

Problem: The document holder is too far off to the side, causing repetitive neck rotation.

Solution: The screen and document holder should be the same distance from the eye (to avoid constant changes of focus), and close enough together so that the operator can look from one to the other without excessive neck or back movement.

Shoulder problems

Shoulder strain can occur when the users arms are positioned too high or too low. When computer operators hands and arms are too high, they tend to pull their shoulders up, straining their shoulder and back muscles. When their hands and arms are too low, they pull their shoulders down, putting pressure on shoulder and back muscles and compressing nerves in the neck and arms.

Problem: The arms are too high or too low when using the keyboard. Solution: Lower the keyboard or raise the chair, and reinforce the principle of keeping the operators hands, arms, and forearms parallel to the keyboard.

Problem: The users arms are too high or too low when using the chair arm-rests.

Solution: Remove the armrests or replace with adjustable armrests. Forearm and hand problems Problems can occur if the users hands dont form a straight line with the forearms, or if the sharp edge of the work surface presses against the palms, wrists, or forearms.

Problem: The keyboard is too thick, too low, or too high, causing wrist bending.

Solution: Purchase thin keyboards to minimize wrist deviation. Adjustable-height and sloped keyboard trays make proper keyboard height and hand-wrist posture easier to accomplish. A keyboard fitted with a wrist rest will support the heel of the operators hand and minimize wrist deviation. The wrist rest thickness should not exceed the height of the first row of keys. Wrists should never rest on the wrist pad when using the keyboard. Wrist rests are to be used between periods of typing.

Problem: The keyboard user supports the wrists on the edge of the work surface while typing or resting. This can cause backward bending of the wrist, numbness of the hand and fingers, or tingling.

Solution: All table surface edges should be rounded, and the keyboard should be retrofitted with a wrist rest.

Leg problems

Leg problems can result from decreased blood circulation. This causes the legs to fall asleep.

Problem: The edge of the seat pan presses against the thighs.

Solutions: A proper seat-pan length allows for a two- to three-finger clearance from the front edge of the chair to the back of the thighs upon properly adjusting the chair height to the workstation. Use a footrest if feet arent flat on the floor.

Problem: Using the foot rungs on the chair or chair legs as footrests can pro-duce excessive knee flexing.

Solutions: Properly adjust chairs and provide a footrest, as needed.

Vision problems

There are no conclusive studies that document permanent vision or eye problems associated with computer use; however, the most common complaints are eye strain, burning eyes, blurred vision, irritated eyes, and headaches.

Solutions: Because these complaints are associated with focusing at close range, the minimum eye distance should be 16 inches from the monitor. Recommend a short rest break (3-5 minutes) following each hour of continuous Computer work, during which time the operator should get up and stretch, move about, or do other work. Periodically focus on distant objects. This serves to relax eye muscles.

Uncorrected or improperly-corrected vision can aggravate any of these complaints. When getting fitted for glasses, computer operators need to tell their eye care specialists that they perform Computer work regularly. The focal distance for reading (10-12 inches) is less than it is for Computer work (16-29 inches).

People wearing bifocals or trifocals have to tilt their heads back to read through the bottom portion of the lenses. This can cause neck strain. Correct the problem by lowering the computer screen height or using single-focal-length glasses specifically for computer use.

Poor or excessive lighting contributes to vision problems. The illumination level for Computer work should be 30-50 footcandles for screen viewing only and 50-70 footcandles for reading printed documents.

Room glare can be reduced or eliminated by lowering the lighting; having the operator sit facing a matte-finished, dark-colored wall; or adjusting the screen upward, downward, or slightly to the left or right. However, too much screen deviation can cause neck problems.

  • Position the Computer workstation at right angles to the window.
  • Install the Computer workstation between rows of overhead lighting.
  • Install screen glare filters and visor hoods over the monitor screen.

This should be your last resort, because it may reduce image quality. Window glare can be reduced or eliminated by covering the windows with draperies or blinds. Blinds are preferred over drapes, and vertical blinds are preferred over horizontal blinds.

  • Install natural-density filter shades over the windows.
  • Add outdoor window awnings.

Muscle fatigue problems

Computer work consists of fixed posture and repetitive motions, resulting in local muscle fatigue. Muscles need adequate rest to prevent discomfort, fatigue, and possible injury, or illness. To reduce local muscle fatigue for computer workers the following are recommended:

  • Take more frequent breaks of shorter duration (3-5 minutes) every hour on the hour.
  • Change job tasks to reduce fatigue and monotony. Allow different sets of muscles to be used with a different posture.
  • Exercise to help relax tight muscles, reduce stress, and lessen the sense of general fatigue.