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The Supreme Value of Adjustability

Adjusting computer workstations improves office workers' comfort and health.

The ability to adapt the workplace to the size variation among computer workers is critical to both promoting users' comfort and protecting their health. While adjustable workstations are commonly thought of in terms of adjusting heights and depths to accommodate differences in sizes of users, it is also important to recognize the importance of adjusting computer workstations to accommodate differences in tasks.

A computer worker who interacts with customers face to face requires a different workstation than one who interacts with customers via telephone. Moreover, the types of tasks performed often vary throughout the workday and the workstation must accommodate, or adjust to accommodate, these changes in tasks in order to maintain user comfort and productivity. Consequently, it is important to consider how to accommodate the variation in tasks the computer user performs, as well as the variation in their own individual characteristics, when designing a computer workstation.

Most Common Types of Discomfort
Visual discomfort, such as dry eyes and headaches, and neck and shoulder musculoskeletal discomfort, such as sore stiff neck and shoulder muscles, are the most common types of discomfort reported by office computer workers. All of these can be alleviated by a mindful arrangement and adjustment of computer workstations.

Three types of adjustments to workstations are effective in alleviating these problems: monitor support and placement, keyboard and mouse placement, and forearm support.

Monitor Support and Positioning
Variation in users and user tasks dictate the need for monitors that adjust vertically and horizontally. Variation in eye height between the tallest and smallest individuals in the U.S. population is in the neighborhood of 9 to 10 inches. This variability is a prime reason for the need for monitor height adjustability. Although the weight of CRT monitors often made this difficult to achieve in the past, the increasing use of lighter-weight flat-panel displays and adjustable mounting solutions now make this readily achievable.

Vertical monitor placement
Vertical monitor placement plays a key role in both visual comfort and in neck and shoulder comfort. A monitor that is placed too high typically causes the user to tilt his or her head and neck back while looking at the monitor; this static extension fatigues the neck and shoulder muscles and quickly becomes uncomfortable. At the same time, looking up at a monitor causes difficulties for the eyes as well as the neck and shoulder muscles. A larger portion of the surface of the eye is exposed, which accelerates drying of the protective, lubricating tear layer. Although the tear layer is renewed as the computer user blinks, the drying effect is compounded by the decreased rate of blinking observed during visually demanding tasks.

It is also more difficult for the eye to focus on near objects (such as computer screens) when they are elevated relative to the level of the eyes and easier to focus on near objects that are below eye level. However, anyone who has attempted to focus on the tip of his or her nose knows firsthand the discomfort associated with focusing on a too-near object, even when it is below eye level! A too-low placement of the monitor also causes the user to tilt his or her neck and head forward; now, it is static flexion of the neck and shoulder muscles that leads to musculoskeletal discomfort.

So what is the optimal solution for vertical monitor placement? Current research1 and technical standards2 recommend that the vertical placement of the monitor be such that the center of the screen is approximately 15 to 25 degrees below horizontal eye level.

This is in good agreement with the commonly used advice to place the top of the viewing area of the screen at or slightly below eye level. For example, the center of a 21-inch diagonal screen viewed at the minimum recommended viewing distance2 of 20 inches would be about 15 to 17.5 degrees below horizontal eye level for screens with 4:3 and 16:9 aspect ratios, respectively, if placed in this manner.

Alternatively, the angle can be estimated based on the horizontal distance between the screen and the users' eyes and the position of the center of the screen below horizontal eye level. At 15 degrees, the center of the screen is about one-fourth of the viewing distance below eye level; at 25 degrees, the center of the screen is slightly less than one-half the viewing distance below horizontal eye level.

Viewing distance
The distance between the monitor and the users' eyes also affects visual comfort and neck and shoulder comfort. Placing the monitor too close to the users' eyes causes discomfort as the eyes turn inward (converge) in order to focus on a near object (the screen) and the ciliary muscles attempt to reshape the lens in order to bring the screen into focus.

While increasing the viewing distance generally improves visual comfort, it is critical that characters displayed on the screen are large enough to be legible at the chosen viewing distance. A monitor that is too far away from the users' eyes to allow the characters displayed on it to be easily read typically results in neck and shoulder discomfort as the user simultaneously extends his or her neck and head while leaning his or her torso forward to decrease the viewing distance to a point where the characters are more legible. The resultant static muscle loading of the shoulder and neck flexor muscles results in discomfort.

The relationship between viewing distance and recommended character size has been well studied. Technical references typically describe recommended character heights for a given viewing distance as a visual angle, for example, ranging between 16 and 22 minutes of arc. Because this can be an unfamiliar unit of measurement, it is often easier to remember and use the James Bond (or "007") rule. In order to be legible, the height of a character should be about 0.007 times the viewing distance. While the height of a 12-point typeface (about one-sixth of an inch) at a 20-inch viewing distance will satisfy this rule and is generally legible, it will likely not be legible for most viewers at a 40-inch viewing distance. Either the typeface or the viewing distance must adjust!

Input Device (Keyboard and Mouse) Position
Just as variation in users' anthropometry and tasks dictates the need for horizontal and vertical adjustment of monitors, so does that same variation dictate a need for adjustable positioning of input devices. Vertical and horizontal positioning of the keyboard and mouse are important contributors to neck and shoulder comfort. For example, too high placement of the keyboard or mouse causes the user to raise his or her shoulders; prolonged keying or mousing in this posture leads to static loading and discomfort in the shoulder muscles. Similarly, research consistently shows that a lack of support to the forearms is also associated with neck and shoulder pain.1,3

Vertical height
An early study of user comfort found that placing input device support surfaces at about elbow height improved user comfort.3 Consistent with that previous research, a recent prospective study4 suggests that placing keyboards and mice at a height between 0 and 3.5 cm (1.4 inches) above the support surface of the level minimizes the risk of neck and shoulder pain.

Height-adjustable support surfaces for input devices are perhaps the most common means of placing keyboards and mice at the appropriate height. Technical design standards, such as ANSI/HFES 100, describe height adjustment range criteria intended to accommodate at least 90 percent of the U.S. adult population. However, in situations where the input device support surface height is fixed, a height-adjustable chair and footrest can be used to place the keyboard and mouse at the user's elbow height.

Horizontal placement
Both reaching too far to the side and too far forward when using the mouse have been associated with shoulder discomfort. Research indicates the input devices should be located in an area about as wide as the users' shoulders and that the maximum shoulder flexion angle when reaching forward while using an input device should not exceed 25 degrees.

Several techniques have been used to bring mice or other pointing devices closer to the keyboard and to place them within the user's shoulder breadth. For users who rarely use the number pad, a compact keyboard without a number pad can be used. An alternate solution is to use a low profile mouse bridge. These devices cover the number pad while mousing and slide out of the way during number pad use.

Summary
Computer users vary in size and shape. The tasks that computer users perform also vary. Workstations that adjust to accommodate the variation in users and tasks promote user health, comfort, and productivity.

References
1. Rempel, et al. (2006), A randomized controlled trial evaluating the effects of two workstation interventions on upper body pain and incident musculoskeletal disorders among computer operators. Occupational and Environmental Medicine, 63, pp. 300-306.
2. HFES (2007) ANSI/HFES 100-2007 Human Factors Engineering of Computer Workstations, Human Factors and Ergonomics Society, www.hfes.org.
3. Berqvist, et al. (1995), Musculoskeletal disorders among visual display terminal workers: individual, ergonomic and work organization factors, Ergonomics, 38, 763-776.
4. Marcus, et al. (2002), A prospective study of computer users: II--Postural risk factors for musculoskeletal symptoms and disorders, American Journal of Industrial Medicine, 41, 236-249.

This article originally appeared in the March 2008 issue of Occupational Health & Safety.

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