HUMAN–COMPUTER INTERACTION:ERGONOMICS

ERGONOMICS

Ergonomics is the science of ﬁtting the environment and activities to the capabilities, dimensions, and needs of people. Ergonomic knowledge and principles are applied to adapt working conditions to the physical, psychological, and social nature of the person. The goal of ergonomics is to improve performance while at the same time enhancing comfort, health, and safety. In particular, the efﬁciency of human–computer interaction, as well as the comfort, health, and safety of users, can be improved by applying ergonomic principles (Grandjean 1979; Smith 1984). However, no simple recommen- dations can be followed that will enhance all of these aspects simultaneously. Compromise is some- times necessary to achieve a set of balanced objectives while ensuring user health and safety (Smith and Sainfort 1989; Smith and Cohen 1997). While no one set of rules can specify all of the necessary combinations of proper working conditions, the use of ergonomic principles and concepts can help in making the right choices.

Components of the Work System

From an ergonomic point of view, the different components of the work system (e.g., the environment, technology, work tasks, work organization, and people) interact dynamically with each other and

function as a total system (see Figure 1). Since changing any one component of the system inﬂuences the other aspects of the system, the objective of ergonomics is to optimize the whole system rather than maximize just one component. In an ergonomic approach, the person is the central focus and the other factors of the work system are designed to help the person be effective, motivated, and comfortable. The consideration of physical, physiological, psychological, and social needs of the person is necessary to ensure the best possible workplace design for productive and healthy human– computer interaction. Table 1 shows ergonomic recommendations for ﬁxed desktop video display terminal (VDT) use that improve the human interface characteristics. Ergonomic conditions for laptop computer use should conform as closely as possible to the recommendations presented in Table 1.

Critical Ergonomics Issues in Human–Computer Interaction

A major feature of the ergonomics approach is that the job task characteristics will deﬁne the er- gonomic interventions and the priorities managers should establish for workplace design requirements. The following discussion of critical areas—the technology, sensory environment, thermal environ- ment, workstation design, and work practices—will highlight the major factors that engineers and managers should be aware of in order to optimize human–computer interaction and protect user health. Speciﬁc recommendations and guidelines will be derived from these discussions, but please be advised that the recommendations made throughout this chapter may have to be modiﬁed to account for differences in technology, personal, situational, or organizational needs at your facility, as well as improved knowledge about human–computer interaction. It cannot be overstated that these considerations represent recommendations and guidelines and not ﬁxed speciﬁcations or standards. The realization that any one modiﬁcation in any single part of the work system will affect the whole system and particularly the person (see Figure 1) is essential for properly applying the following recommendations and speciﬁcations.

Ergonomics of Computer Interfaces

Today, the primary display interfaces in human–computer interaction are the video display with a cathode ray tube and the ﬂat panel screen. In the early 1980s, the US Centers for Disease Control (CDC 1980) and the U.S. National Academy of Sciences deﬁned important design considerations for the use of cathode ray tubes (NAS 1983). The Japan Ergonomics Society (JES) established a Com- mittee for Flat Panel Display Ergonomics in 1996, which proposed ergonomic guidelines for use of products with ﬂat panels, such as liquid crystal displays (LCDs) (JES 1996). These Japanese guide- lines were subsequently reviewed by the Committee on Human–Computer Interaction of the Inter- national Ergonomics Association (IEA). The JES guidelines addressed the following issues: (1) light-related environmental factors, (2) device use and posture factors, (3) environmental factors, (4) job design factors, and (5) individual user factors. These guidelines will be discussed in appropriate sections of this chapter.

The use of CRTs and ﬂat panel displays has been accompanied by user complaints of visual fatigue, eye soreness, general visual discomfort, and various musculoskeletal complaints and discom- fort with prolonged use (Grandjean 1979; Smith et al. 1981; NIOSH 1981; NAS 1983; Smith 1984; JES 1996). Guidelines for providing the proper design of the VDT and the environment in which it is used have been proposed by the Centers for Disease Control (CDC 1980) and the Human Factors and Ergonomics Society (ANSI 1988), and for the laptop and palm computer, by the Japan Ergon- omics Society (JES 1996). The following sections deal with the visual environment for using desktop computers, but the discussion can be extrapolated to other types of computer use.

The major interfaces of employees with computers are the screen (CRT, ﬂat panel), the keyboard, and the mouse. Other interfaces are being used more and more, such as voice input, pointers, hand- actuated motion devices, and apparatuses for virtual environment immersion.

The Screen and Viewing

Poor screen images, ﬂuctuating and ﬂickering screen luminances, and screen glare cause user visual discomfort and fatigue (Grandjean 1979; NAS 1983). There are a range of issues concerning read- ability and screen reﬂections. One is the adequacy of contrast between the characters and screen background. Screens with glass surfaces have a tendency to pick up glare sources in the environment and reﬂect them. This can diminish the contrast of images on the screen. To reduce environmental glare, the luminance ratio within the user’s near ﬁeld of vision should be approximately 1:3, and within the far ﬁeld approximately 1:10 (NIOSH 1981). For luminance on the screen itself, the char- acter-to-screen background luminance contrast ratio should be at least 7:1 (NIOSH 1981). To give the best readability for each operator, it is important to provide VDTs with adjustments for character contrast and brightness. These adjustments should have controls that are obvious to observe and manipulate and easily accessible from normal working position (e.g., located at the front of the screen) (NIOSH 1981).

Good character design can help improve image quality, which is a major factor for reducing eyestrain and visual fatigue. The proper size of a character is dependent on the task and the display parameters (brightness, contrast, glare treatment, etc.) and the viewing distance. Character size that is too small

can make reading difﬁcult and cause the visual focusing mechanism to overwork. This produces eyestrain and visual fatigue (NAS 1983). Character heights should preferably be at least 20–22 min of visual arc, while character width should be 70–80% of the character height (Smith 1984; ANSI 1988). This approximately translates into a minimum lowercase character height of 3.5 mm with a width of 2.5 mm at a normal viewing distance of 50 cm.

Good character design and proper horizontal and vertical spacing of characters can help improve image quality. To ensure adequate discrimination between characters and good screen readability, the character spacing should be in the range of 20–50% of the character width. The interline spacing should be 50–100% of the character height (Smith 1984; ANSI 1988).

The design of the characters inﬂuences their readability. Some characters are hard to decipher, such as lowercase g, which looks like numeral 8. A good font design minimizes character confusion and enhances the speed at which characters can be distinguished and read. Two excellent fonts are Huddleston and Lincoln-Mitre (NAS 1983). Most computers have a large number of fonts to select from. Computer users should choose a font that is large enough to be easy for them to read.

Viewing Distance

Experts have traditionally recommended a viewing distance between the screen and the operator’s eye of 45–50 cm but no more than 70 cm (Grandjean 1979; Smith 1984). However, experience in ﬁeld studies has shown that users may adopt a viewing distance greater than 70 cm and are still able to work efﬁciently and not develop visual problems. Thus, viewing distance should be determined in context with other considerations. It will vary depending on the task requirements, CRT screen characteristics, and individual’s visual capabilities. For instance, with poor screen or hard copy qual- ity, it may be necessary to reduce viewing distances for easier character recognition. Typically, the viewing distance should be 50 cm or less due to the small size of characters on the VDT screen. LNCs are often used in situations where the computer is placed on any convenient surface, for example a table at the airport waiting room. Thus, the viewing distance is deﬁned by the available surface, not a ﬁxed workstation. When the surface is farther away from the eyes, the font size used should be larger.

Proper viewing distance will be affected by the condition of visual capacity and by the wearing of spectacles / lenses. Persons with myopia (near-sightedness) may ﬁnd that they want to move the screen closer to their eyes; while persons with presbyopia (far-sightedness) or bifocal lenses may want the screen farther away from their eyes. Many computer users who wear spectacles have a special pair of spectacles with lenses that are matched to their particular visual defect and a com- fortable viewing distance to the screen. Eyecare specialist can have special spectacles made to meet computer users’ screen use needs.

Screen Flicker and Image Stability

The stability of the screen image is another characteristic that contributes to CRT and LCD quality. Ideally, the display should be completely free of perceptible movements such as ﬂicker or jitter (NIOSH 1981). CRT screens are refreshed a number of times each second so that the characters on the screen appear to be solid images. When this refresh rate is too low, users perceive screen ﬂicker. LCDs have less difﬁculty with ﬂicker and image stability than CRT displays. The perceptibility of screen ﬂicker depends on illumination, screen brightness, polarity, contrast, and individual sensitivity. For instance, as we get older and our visual acuity diminishes, so too does our ability to detect ﬂicker. A screen with a dark background and light characters has less ﬂicker than screens with dark lettering on a light background. However, light characters on a dark background show more glare. In practice, ﬂicker should not be observable, and to achieve this a screen refresh rate of at least 70 cycles per second needs to be achieved for each line on the CRT screen (NAS 1983; ANSI 1988). With such a refresh rate, ﬂicker should not be a problem for either screen polarity (light on dark or dark on light). It is a good idea to test a screen for image stability. Turn the lights down, increase the screen brightness / contrast settings, and ﬁll the screen with letters. Flickering of the entire screen or jitter of individual characters should not be perceptible, even when viewed peripherally.

Screen Swivel and Tilt

Reorientation of the screen around its vertical and horizontal axes can reduce screen reﬂections and glare. Reﬂections can be reduced by simply tilting the display slightly back or down or to the left or right, depending on the angle of the source of glare. These adjustments are easiest if the screen can be tilted about its vertical and horizontal axes. If the screen cannot be tilted, it should be approximately vertical to help eliminate overhead reﬂections, thus improving legibility and posture.

The perception of screen reﬂection is inﬂuenced by the tilt of the screen up or down and back and forth and by the computer user’s line of sight toward the screen. If the screen is tilted toward sources of glare and these are in the computer user’s line of sight to the screen, the screen images will have poorer clarity and reﬂections can produce disability glare (see Section 2.4.4). In fact, the line of sight can be a critical factor in visual and musculoskeletal discomfort symptoms. When the line of sight can observe glare or reﬂections, then eyestrain often occurs. For musculoskeletal con- siderations, experts agree that the line of sight should never exceed the straight-ahead horizontal gaze, and in fact it is best to provide a downward gaze of about 10–20° from the horizontal when viewing the top of the screen and about 40° when viewing the bottom edge of the screen (NIOSH 1981; NAS 1983; Smith 1984; ANSI 1988). This will help reduce neck and shoulder fatigue and pain. These gaze considerations are much harder to obtain when using LNCs because of the smaller screen size and workstation features (eg., airport waiting room table).

The Visual Environment

Lighting

Lighting is an important aspect of the visual environment that inﬂuences readability and glare on the screen and viewing in the general environment. There are four types of general workplace illumination of interest to the computer user’s environment:

1. Direct radiants: The majority of ofﬁce lighting is direct radiants. These can be incandescent lights, which are most common in homes, or ﬂuorescent lighting, which is more prevalent in workplaces and stores. Direct radiants direct 90% or more of their light toward the object(s) to be illuminated in the form of a cone of light. They have a tendency to produce glare.

2. Indirect lighting: This approach uses reﬂected light to illuminate work areas. Indirect lighting directs 90% or more of the light onto the ceiling and walls, which reﬂect it back into the room. Indirect lighting has the advantage of reducing glare, but supplemental lighting is often nec- essary, which can be a source of glare.

3. Mixed direct radiants and indirect lighting: In this approach, part of the light (about 40%) radiates in all directions while the rest is thrown directly or indirectly onto the ceiling and walls.

4. Opalescent globes: These lights give illumination equally in all directions. Because they are bright, they often cause glare.

Modern light sources used in these four general approaches to workplace illumination are typically of two kinds: electric ﬁlament lamps and ﬂuorescent tubes. Following are the advantages and draw- backs of these two light sources:

1. Filament lamps: The light from ﬁlament lamps is relatively rich in red and yellow rays. It changes the apparent colors of objects and so is unsuitable when correct assessment of color is essential. Filament lamps have the further drawback of emitting heat. On the other hand, employees like their warm glow, which is associated with evening light and a cozy atmosphere.

2. Fluorescent tubes: Fluorescent lighting is produced by passing electricity through a gas. Flu- orescent tubes usually have a low luminance and thus are less of a source of glare. They also have the ability to match their lighting spectrum to daylight, which many employees ﬁnd preferable. They may also be matched to other spectrums of light that can ﬁt ofﬁce decor or employee preferences. Standard-spectrum ﬂuorescent tubes are often perceived as a cold, pale light and may create an unfriendly atmosphere. Fluorescent tubes may produce ﬂicker, espe- cially when they become old or defective.

Illumination

The intensity of illumination or the illuminance being measured is the amount of light falling on a surface. It is a measure of the quantity of light with which a given surface is illuminated and is measured in lux. In practice, this level depends on both the direction of ﬂow of the light and the spatial position of the surface being illuminated in relation to the light ﬂow. Illuminance is measured in both the horizontal and vertical planes. At computer workplaces, both the horizontal and vertical illuminances are important. A document lying on a desk is illuminated by the horizontal illuminance, whereas the computer screen is illuminated by the vertical illuminance. In an ofﬁce that is illuminated from overhead luminaires, the ratio between the horizontal and vertical illuminances is usually be- tween 0.3 and 0.5. So if the illuminance in a room is said to be 500 lux, the horizontal illuminance is 500 lux while the vertical illuminance is between 150 and 250 lux (0.3 and 0.5 of the horizontal illuminance).

The illumination required for a particular task is determined by the visual requirements of the task and the visual ability of the employees concerned. In general, an illuminance in the range of 300–700 lux measured on the horizontal working surface (not the computer screen) is normally preferable (CDC 1980; NAS 1983). The JES (1996) recommends ofﬁce lighting levels ranging from 300–1,000 lux for ﬂat panel displays. Higher illumination levels are necessary to read hard copy and lower illumination levels are better for work that just uses the computer screen. Thus, a job in which hard copy and a computer screen are both used should have a general work area illumination level of about 500–700 lux, while a job that only requires reading the computer screen should have a general work area illumination of 300–500 lux. Conﬂicts can arise when both hardcopy and computer screens are used by different employees who have differing job task requirements or differing visual capabilities and are working in the same area. As a compromise, room lighting can be set at the recommended lower (300 lux) or intermediate level (500 lux) and additional task lighting can be provided as needed. Task lighting refers to localized lighting at the workstation to replace or sup- plement ambient lighting systems used for more generalized lighting of the workplace. Task lighting is handy for illuminating hardcopy when the room lighting is set at a low level, which can hinder document visibility. Such additional lighting must be carefully shielded and properly placed to avoid glare and reﬂections on the computer screens and other adjacent working surfaces of other employees. Furthermore, task lighting should not be too bright in comparison to the general work area lighting since looking between these two different light levels may produce eyestrain.

Luminance

Luminance is a measure of the brightness of a surface, that is, the amount of light leaving the surface of an object, either reﬂected by the surface (as from a wall or ceiling), emitted by the surface (as from the CRT or LCD characters), or transmitted (as light from the sun that passes through translucent curtains). Luminance is expressed in units of candelas per square meter. High-intensity luminance sources (such as windows) in the peripheral ﬁeld of view should be avoided. In addition, the balance among the luminance levels within the computer user’s ﬁeld of view should be maintained. The ratio of the luminance of a given surface or object to another surface or object in the central ﬁeld of vision should be around 3:1, while the luminance ratio in the peripheral ﬁeld of vision can be as high as 10:1 (NAS 1983).

Glare

Large differences in luminance or high-luminance lighting sources can cause glare. Glare can be classiﬁed with respect to its effects (disability glare vs. discomfort glare) or the source of glare (direct glare vs. reﬂected glare). Glare that results in an impairment of vision (e.g., reduction of visual acuity) is called disability glare, while discomfort glare is experienced as a source of discomfort to the viewer but does not necessarily interfere with visual performance. With regard to the source, direct glare is caused by light sources in the ﬁeld of view of the computer user, while reﬂected glare is caused by reﬂections from illuminated, polished, or glossy surfaces or by large luminance differ- ences in the visual environment. In general, glare is likely to increase with the luminance, size, and proximity of the lighting source to the line of sight.

Direct and reﬂected glare can be limited through one or more of the following techniques:

1. Controlling the light from windows: This can be accomplished by closing drapes, shades, and/ or blinds over windows or awnings on the outside, especially during sunlight conditions.

2. Controlling the view of luminaires:

(a) By proper positioning of CRT screen with regard to windows and overhead lighting to reduce direct or reﬂected glare and images. To accomplish this, place VDTs parallel to windows and luminaires and between luminaires rather than underneath them.

(b) Using screen hoods to block luminaires from view.

(c) Recessing light ﬁxtures.

(d) Using light-focusing diffusers.

3. Controlling glare at the screen surface by:

(a) Adding antiglare ﬁlters on the VDT screen.

(b) Proper adjustment up or down / left or right of the screen.

4. Controlling the lighting sources using:

(a) Appropriate glare shields or covers on the lamps.

(b) Properly installed indirect lighting systems.

Glare can also be caused by reﬂections from surfaces, such as working surfaces, walls, or the ﬂoor covering. These surfaces do not emit light themselves but can reﬂect it. The ratio of the amount of light reﬂected by a surface (luminance) to the amount of light striking the surface (illuminance) is called reﬂectance. Reﬂectance is unitless. The reﬂectance of the working surface and the ofﬁce

machines should be on the order of 40–60% (ANSI 1988). That is, they should not reﬂect more than 60% of the illuminance striking their surface. This can be accomplished if surfaces have a matte ﬁnish.

Generally, ﬂoor coverings should have a reﬂectance of about 30%, ceilings, of 80–90%, and walls, 40–60%. Reﬂectance should increase from the ﬂoor to the ceiling. Although the control of surface reﬂections is important, especially with regard to glare control, it should not be at the expense of a pleasant working environment where employees feel comfortable. Walls and ceilings should not be painted dark colors just to reduce light reﬂectance, nor should windows be completely covered or bricked up to keep out sunlight. Other, more reasonable luminance control approaches can give positive beneﬁts while maintaining a psychologically pleasing work environment.

The Auditory Environment

Noise

A major advantage of computer technology over the typewriter is less noise at the workstation. However, it is not unusual for computer users to complain of bothersome ofﬁce noise, particularly from ofﬁce conversation. Noise levels commonly encountered in ofﬁces are below established limits that could cause damage to hearing (i.e., below 85 dBA). The JES (1996) proposed that the noise level should not exceed 55 dBA. The expectations of ofﬁce employees are for quiet work areas because their tasks often require concentration. Annoying noise can disrupt their ability to concentrate and may produce stress.

Actually, there are many sources of annoyance noise in computer operations. Fans in computers, printers, and other accessories, which are used to maintain a favorable internal device temperature, are a source of noise. Ofﬁce ventilation fans can also be a source of annoyance noise. The computers themselves may be a source of noise (e.g., the click of keys or the high-pitched squeal of the CRT). The peripheral equipment associated with computers, such as printers, can be a source of noise. Problems of noise may be exacerbated in open-plan ofﬁces, in which noise is harder for the individual employee to control than in enclosed ofﬁces.

Acoustical control can rely upon ceiling, ﬂoor and wall, furniture, and equipment materials that absorb sound rather than reﬂect it. Ceilings that scatter, absorb, and minimize the reﬂection of sound waves are desirable to promote speech privacy and reduce general ofﬁce noise levels. The most common means of blocking a sound path is to build a wall between the source and the receiver. Walls are not only sound barriers but are also a place to mount sound-absorbent materials. In open- plan ofﬁces, free-standing acoustical panels can be used to reduce the ambient noise level and also to separate an individual from the noise source. Full effectiveness of acoustical panels is achieved in concert with the sound-absorbent materials and ﬁnishes applied to the walls, ceiling, ﬂoor, and other surfaces. For instance, carpets not only cover the ﬂoor but also serve to reduce noise. This is achieved in two ways: (1) carpets absorb the incident sound energy and (2) gliding and shufﬂing movements on carpets produce less noise than on bare ﬂoors. Furniture and draperies are also important for noise reduction.

Acoustical control can also be achieved by proper space planning. For instance, workstations that are positioned too closely do not provide suitable speech privacy and can be a source of disturbing conversational noise. As a general rule, a minimum of 8–10 ft between employees, separated by acoustical panels or partitions, will provide normal speech privacy.

Heating, Ventilating, and Air Conditioning (HVAC )

Temperature, humidity, air ﬂow, and air exchanges are important parameters for employees’ perform- ance and comfort.

It is unlikely that ofﬁces will produce excessive temperatures that could be physically harmful to employees. However, thermal comfort is an important consideration in employee satisfaction that can inﬂuence performance. Satisfaction is based not on the ability to tolerate extremes but on what makes an individual happy. Many studies have shown that most ofﬁce employees are not satisﬁed with their thermal comfort. The deﬁnition of a comfortable temperature is usually a matter of personal pref- erence. Opinions as to what is a comfortable temperature vary within an individual from time to time and certainly among individuals. Seasonal variations of ambient temperature inﬂuence perceptions of thermal comfort. Ofﬁce employees sitting close to a window may experience the temperature as being too cold or hot, depending on the outside weather. It is virtually impossible to generate one room temperature in which all employees are equally well satisﬁed over a long period of time.

As a general rule, it is recommended that the temperature be maintained in the range of 20–24°C (68–75°F) in winter and 23–27°C (73–81°F) in summer (NIOSH 1981; Smith 1984). The JES (1996) recommends ofﬁce temperatures of 20–23°C in winter and 24–27°C in summer.

Air ﬂows across a person’s neck, head, shoulders, arms, ankles, and knees should be kept low (below 0.15 m / sec in winter and below 0.25 m / sec in summer). It is important that ventilation not produce currents of air that blow directly on employees. This is best handled by proper placement of the workstation.

Relative humidity is an important component of ofﬁce climate and inﬂuences an employee’s comfort and well being. Air that is too dry leads to drying out of the mucous membranes of the eyes, nose, and throat. Individuals who wear contact lenses may be made especially uncomfortable by dry air. In instances where intense, continuous near-vision work at the computer is required, very dry air has been shown to irritate the eyes. As a general rule, it is recommended that the relative humidity in ofﬁce environments be at least 50% and less than 60% (NIOSH 1981; Smith 1984). The JES (1996) recommends humidity levels of 50–60%. Air that is too wet enhances the growth of unhealthy organisms (molds, fungus, bacteria) that can cause disease (legionnaires’, allergies).

Computer Interfaces

Computer interfaces are the means by which users provide instructions to the computer. There are a wide variety of devices for interfacing, including keyboards, mice, trackballs, joy sticks, touch panels, light pens, pointers, tablets, and hand gloves. Any mechanical or electronic device that can be tied to a human motion can serve as a computer interface. The most common interfaces in use today are the keyboard and the mouse. The keyboard will be used as an example to illustrate how to achieve proper human–computer interfaces.

The Keyboard

In terms of computer interface design, a number of keyboard features can inﬂuence an employee’s comfort, health, and performance. The keyboard should be detachable and movable, thus providing ﬂexibility for independent positioning of the keyboard and screen. This is a major problem with LNCs because the keyboard is built into the top of the computer case for portability and convenience. It is possible to attach a separate, detachable keyboard to the LNC, and this should be done when the LNC is used at a ﬁxed workstation in an ofﬁce or at home. Clearly, it would be difﬁcult to have a separate keyboard when travelling and the LNC portability feature is paramount.

The keyboard should be stable to ensure that it does not slide on the tabletop. This is a problem when an LNC is held in the user’s lap or some other unstable surface. In order to help achieve a favorable user arm height positioning, the keyboard should be as thin as possible. The slope or angle of the keyboard should be between 0° and 15°, measured from the horizontal. LNCs are limited in keyboard angle because the keyboard is often ﬂat (0°). However, some LNCs have added feet to the computer case to provide an opportunity to increase the keyboard angle. Adjustability of keyboard angle is recommended. While the ANSI standard (ANSI 1988) suggests 0–25°, we feel angles over 15° are not necessary for most activities.

The shape of the key tops must satisfy several ergonomic requirements, such as minimizing reﬂections, aiding the accurate location of the operator’s ﬁnger, providing a suitable surface for the key legends, preventing the accumulation of dust, and being neither sharp nor uncomfortable when depressed. For instance, the surface of the key tops, as well as the keyboard itself, should have a matte ﬁnish. The key tops should be approximately 200 mm (ANSI 1988) with a minimum horizontal width of 12 mm. The spacing between the key centers should be about 18–19 mm horizontally and 18–20 mm vertically (ANSI 1988). There should be slight protrusions on select keys on the home row to provide tactile information about ﬁnger position on the keyboard.

The force to depress the key should ideally be between 0.5 N and 0.6 N (ANSI 1988). However, ranges from 0.25–1.5 N have been deemed acceptable (ANSI 1988). The HFES / ANSI-100 standard is currently being revised, and this recommendation may change soon. Some experts feel that the keying forces should be as low as feasible without interfering with motor coordination. Research has shown that light-touch keys require less operator force in depressing the key (Rempel and Gerson, 1991; Armstrong et al. 1994; Gerard et al. 1996). The light-touch force keyboards vary between 0.25–0.40 N.

Feedback from typing is important for beginning typists because it can indicate to the operator that the keystroke has been successfully completed. There are two main types of keyboard feedback: tactile and auditory. Tactile feedback can be provided by a collapsing spring that increases in tension as the key is depressed or by a snap-action mechanism when key actuation occurs. Auditory feedback (e.g., ‘‘click’’ or ‘‘beep’’) can indicate that the key has been actuated. Of course, there is also visual feedback on the computer screen. For experienced typists, the feedback is not useful, as their ﬁngers are moving in a ballistic way that is too fast for the feedback to be useful for modifying ﬁnger action (Guggenbuhl and Krueger 1990, 1991; Rempel and Gerson 1991; Rempel et al. 1992).

The keyboard layout can be the same as that of a conventional typewriter, that is, the QWERTY design, or some other proven style, such as the DVORAK layout. However, it can be very difﬁcult for operators to switch between keyboards with different layouts. Traditional keyboard layout has straight rows and staggered columns. Some authors have proposed curving the rows to provide a better ﬁt for the hand to reduce biomechanical loading on the ﬁngers (Kroemer 1972). However, there is no research evidence that such a design provides advantages for operator’s performance or health.

Punnett and Bergqvist (1997) have proposed that keyboard design characteristics can lead to upper-extremity musculoskeletal disorders. There is controversy about this contention by Punnett and Bergqvist because there are many factors involved in computer typing jobs independent of the key- board characteristics that may contribute to musculoskeletal disorders. Some ergonomists have de- signed alternative keyboards in attempts to reduce the potential risk factors for musculoskeletal disorders (Kroemer 1972; Nakaseko et al. 1985; Ilg 1987). NIOSH (1997) produced a publication that describes various alternative keyboards. Studies have been undertaken to evaluate some of these alternative keyboards (Swanson et al. 1997; Smith et al. 1998). The research results indicated some improvement in hand / wrist posture from using the alternative keyboards, but no decrease in mus- culoskeletal discomfort.

Accessories

The use of a wrist rest when keying can help to minimize extension (backward bending) of the hand. A wrist rest should have a fairly broad surface (approximately 5 cm) with a rounded front edge to prevent cutting pressures on the wrist and hands. Padding further minimizes skin compression and irritation. Height adjustability is important so that the wrist rest can be set to a preferred level in concert with the keyboard height and slope. Some experts are concerned that resting the wrist on a wrist rest during keying could cause an increase in intercarpal canal pressure. They prefer that wrist rests be used only when the user is not keying for the purpose of resting the hands and wrist. Thus, they believe users need to be instructed (trained) about when and how to use a wrist rest. Arm holders are also available to provide support for the hands, wrists, and arms while keyboarding. However, these may also put pressure on structures that may produce nerve compression. As with a wrist rest, some experts feel these devices are best used only during rest from keying.

The Mouse

The most often-used computer pointing device is the mouse. While there are other pointing devices, such as the joystick, touch panel, trackball, and light pen, the mouse is still the most universally used of these devices. An excellent discussion of these pointing devices can be found in Bullinger et al. (1977). The mouse provides for an integration of both movement of the cursor and action on computer screen objects, simultaneously. Many mice have multiple buttons to allow for several actions to occur in sequence. The ease of motion patterns and multiple-function buttons give the mouse an advantage over other pointing devices. However, a disadvantage of the mouse is the need for tabletop space to achieve the movement function. Trankle and Deutschmann (1991) conducted a study to determine which factors inﬂuenced the speed of properly positioning a cursor with a mouse. The results indi- cated that the most important factors were the target size and the distance traveled. Also of lesser importance was the display size arc. The control / response ratio or the sensitivity of the control to movement was not found to be important. Recently, studies have indicated that operators have re- ported musculoskeletal discomfort due to mouse use (Karlqvist et al. 1994; Armstrong et al. 1995; Hagberg 1995; Fogelman and Brogmus 1995; Wells et al. 1997).

The Workstation

Workstation design is a major element in ergonomic strategies for improving user comfort and par- ticularly for reducing musculoskeletal problems. Figure 2 illustrates the relationships among the working surface, VDT, chair, documents, and various parts of the body. Of course, this is for a ﬁxed workstation at the ofﬁce or home. Use of LNCs often occurs away from ﬁxed workstations where it is difﬁcult to meet the requirements described below. However, efforts should be made to meet these requirements as much as possible, even when using LNCs.

The task requirements will determine critical layout characteristics of the workstation. The relative importance of the screen, keyboard, and hard copy (i.e., source documents) depends primarily on the task, and this deﬁnes the design considerations necessary to improve operator performance, comfort, and health. Data-entry jobs, for example, are typically hard copy oriented. The operator spends little time looking at the screen, and tasks are characterized by high rates of keying. For this type of task it is logical for the layout to emphasize the keyboard, mouse, and hard copy, because these are the primary tools used in the task, while the screen is of lesser importance. On the other hand, data- acquisition operators spend most of their time looking at the screen and seldom use hard copy. For this type of task, the screen and the keyboard layout should be emphasized.

Working Surfaces

The size of the work surface is dependent on the task(s), documents, and technology. The primary working surface (e.g., supporting the keyboard, display, and documents) should be sufﬁcient to: (1) permit the screen to be moved forward or backward to a comfortable viewing distance for a range

of users, (2) allow a detachable keyboard to be placed in several locations, and (3) permit source documents to be properly positioned for easy viewing. Additional working surfaces (i.e., secondary working surfaces) may be required in order to store, lay out, read, and / or write on documents or materials. Often users have more than one computer, so a second computer is placed on a secondary working surface. In such a situation, workstations are conﬁgured so that multiple pieces of equipment and source materials can be equally accessible to the user. In this case, additional working surfaces are necessary to support these additional tools and are arranged to allow easy movement while seated from one surface to another.

The tabletop should be as thin as possible to provide clearance for the user’s thighs and knees. Moreover, it is important to provide unobstructed room under the working surface for the feet and legs so that users can easily shift their posture. Knee space height and width and leg depth are the three key factors for the design of clearance space under working surfaces (see Figure 2). Recom- mendations for minimum width for leg clearance is 51 cm, while the preferred minimum width is 61 cm (ANSI, 1988). The minimum depth under the work surface from the operator edge of the work surface should be 38 cm for clearance at the knee level and 60 cm at the toe level (ANSI 1988). A good workstation design accounts for individual body sizes and often exceeds minimum clearances to allow for free postural movement.

Table height has been shown to be an important contributor to computer user musculoskeletal problems. In particular, tables that are too high cause the keyboard to be too high for many operators. The standard desk height of 30 in. (76 cm) is often too high for most people to attain the proper arm angle when using the keyboard. This puts undue pressure on the hands, wrists, arms, shoulders, and neck. It is desirable for table heights to vary with the trunk height of the operator. Height- adjustable tables are effective for this. Adjustable multisurface tables enable good posture by allowing the keyboard and display to be independently adjusted to appropriate keying and viewing heights for each individual and each task. Tables that cannot be adjusted easily are not appropriate when used by several individuals of differing sizes. If adjustable tables are used, ease of adjustment is essential. Adjustments should be easy to make and operators should be instructed (trained) about how to adjust the workstation to be comfortable and safe.

Speciﬁcations for the height of working surfaces vary by whether the table is adjustable or ﬁxed in height and depending on a single working surface or multiple working surfaces. Remember that adjustable-height working surfaces are strongly recommended. However, if the working surface height is not adjustable, the proper height for a nonadjustable working surface is about 70 cm (ﬂoor to top of surface) (ANSI 1988). Adjustable tables allow vertical adjustments of the keyboard and display. Some allow for independent adjustment of the keyboard and display. For single adjustable working surfaces, the working surface height adjustment should be 70–80 cm. For independently adjustable working surfaces for the keyboard and screen, the appropriate height range for the keyboard surface is 59–71 cm, and 70–80 cm for the screen (ANSI 1988).

The Chair

Poorly designed chairs can contribute to computer user discomfort. Chair adjustability in terms of height, seat angle, lumbar support, and armrest height and angle reduces the pressure and loading on the musculoskeleture of the back, legs, shoulders, neck, and arms. In addition, how the chair supports the movement of the user (the chair’s action) helps to maintain proper seated posture and encourages good movement patterns. A chair that provides swivel action encourages movement, while backward tilting increases the number of postures that can be assumed. The chair height should be adjustable so that the feet can rest ﬁrmly on the ﬂoor with minimal pressure beneath the thighs. The minimum range of adjustment for seat height should be 38–52 cm (NAS 1983; Smith 1984; ANSI 1988). Modern chairs also provide an action that supports the back (spine) when seated. Examples of such chairs are the Leap by Steelcase, Inc. and the Aeron by Herman Miller.

To enable shorter users to sit with their feet on the ﬂoor without compressing their thighs, it may be necessary to add a footrest. A well-designed footrest has the following features: (1) it is inclined upward slightly (about 5–15°), (2) it has a nonskid surface, (3) it is heavy enough that it does not slide easily across the ﬂoor, (4) it is large enough for the feet to be ﬁrmly planted, and (5) it is portable.

The seat pan is where the user’s buttocks sits on the chair. It is the part that directly supports the weight of the buttocks. The seat pan should be wide enough to permit operators to make slight shifts in posture from side to side. This not only helps to avoid static postures but also accommodates a large range of individual buttock sizes with a few seat pan widths. The minimum seat pan width should be 45 cm and the minimum depth 38–43 cm (ANSI 1988). The front edge of the chair should be well rounded downward to reduce pressure on the underside of the thighs, which can affect blood ﬂow to the legs and feet. The seat needs to be padded to the proper ﬁrmness that ensures an even distribution of pressure on the thighs and buttocks. A properly padded seat should compress about one-half to one inch when a person sits on it.

Some experts feel that the seat front should be elevated slightly (up to 7°), while others feel it should be lowered slightly (about 5°) (ANSI 1988). There is little agreement among the experts about which is correct (Grandjean 1979, 1984). Many chair manufacturers provide adjustment of the front angle so the user can have the preferred tilt angle, either forward or backward.

The tension for leaning backward and the backward tilt angle of the backrest should be adjustable. Inclination of chair backrest is important for users to be able to lean forward or back in a comfortable manner while maintaining a correct relationship between the seat pan angle and the backrest incli- nation. A back seat inclination of about 110° is considered as the best position by many experts (Grandjean 1984). However, studies have shown that operators may incline backward as much as 125°. Backrests that tilt to allow an inclination of up to 125–130° are a good idea. The advantage of having an independent tilt angle adjustment is that the backrest tilt will then have little or no effect on the front seat height. This also allows operators to shift postures easily and often.

Chairs with full backrests that provide lower back (lumbar) support and upper back (lower shoul- der) support are preferred. This allows employees to lean backward or forward, adopting a relaxed posture and resting the back muscles. A full backrest with a height around 45–51 cm is recommended (ANSI 1988). However, some of the newer chair designs do not have the bottom of the backrest go all the way to the seat pan. This is acceptable as long as the lumbar back is properly supported. To prevent back strain with such chairs, it is recommended that they have midback (lumbar) support since the lumbar region is one of the most highly loaded parts of the spine.

For most computer workstations, chairs with rolling castors (or wheels) are desirable. They are easy to move and facilitate the postural adjustment of users, particularly when the operator has to access equipment or materials that are on secondary working surfaces. Chairs should have a ﬁve-star base for tipping stability (ANSI 1988).

Another important chair feature is armrests. Pros and cons for the use of armrests at computer workstations have been advanced. On the one hand, some chair armrests can present problems of restricted arm movement, interference with keyboard operation, pinching of ﬁngers between the armrest and table, restriction of chair movement such as under the work table, irritation of the arm or elbows, and adoption of awkward postures.

On the other hand, well-designed armrests or elbow rests can provide support for resting the arms to prevent or reduce fatigue, especially during breaks from typing. Properly designed armrests can overcome the problems mentioned because they can be raised, lowered, and angled to ﬁt the user’s needs. Removable armrests are an advantage because they provide greater ﬂexibility for individual user preference, especially for users who develop discomfort and pain from the pressure of the armrest on their arms.

Other Workstation Considerations

An important component of the workstation that can help reduce musculoskeletal loading is a doc- ument holder. When properly designed and proportioned, document holders reduce awkward incli-

nations, as well as frequent movements up and down and back and forth of the head and neck. They permit source documents to be placed in a central location at approximately the same viewing distance and height as the computer screen. This eliminates needless head and neck movements and reduces eyestrain. In practice, some ﬂexibility about the location, adjustment, and position of the document holder should be maintained to accommodate both task requirements and operator preferences. The document holder should have a matte ﬁnish so that it does not produce reﬂections or a glare source.

Privacy requirements include both visual and acoustical control of the workplace. Visual control prevents physical intrusions and distractions, contributes to protecting conﬁdential / private conver- sations, and prevents the individual from feeling constantly watched. Acoustical control prevents distracting and unwanted noise—from machine or conversation—and permits speech privacy. While certain acoustical methods and materials such as free-standing panels are used to control general ofﬁce noise level, they can also be used for privacy. In open-ofﬁce designs they can provide work- station privacy. Generally, noise control at a computer workstation can be achieved through the following methods:

• Use of vertical barriers, such as acoustical screens or panels.

• Selection of ﬂoor, ceiling, wall, and workstation materials and ﬁnishes according to their power to control noise.

• Placement of workstations to enhance individual privacy.

• Locating workstations away from areas likely to generate noise (e.g., printer rooms, areas with heavy trafﬁc).

Each of these methods can be used individually or combined to account for the speciﬁc visual and acoustical requirements of the task or individual employee needs. Planning for privacy should not be made at the expense of visual interest or spatial clarity. For instance, providing wide visual views can prevent the individual from feeling isolated. Thus, a balance between privacy and openness enhances user comfort, work effectiveness, and ofﬁce communications. Involving the employee in decisions of privacy can help in deciding the compromises between privacy and openness.

Work Practices

Good ergonomic design of computer workstations has the potential to reduce visual and musculo- skeletal complaints and disorders as well as increase employee performance. However, regardless of how well a workstation is designed, if operators must adopt static postures for a long time, they can still have performance, comfort, and health problems. Thus, designing tasks that induce employee movement in addition to work breaks can contribute to comfort and help relieve employees’ fatigue.

Work Breaks

As a minimum, a 15-minute break from working should be taken after 2 hours of continuous computer work (CDC 1980; NIOSH 1981). Breaks should be more frequent as visual, muscular, and mental loads are high and as users complain of visual and musculoskeletal discomfort and psychological stress. With such intense, high-workload tasks, a work break of 10 minutes should be taken after 1 hour of continuous computer work. More frequent breaks for alternative work that does not pose demands similar to the primary computer work can be taken after 30 minutes of continuous computer work. Rest breaks provide an opportunity for recovery from local visual, musculoskeletal, and mental fatigue, to break from monotonous activities, or to engage in activities that provide variety in sensory, motor, and cognitive requirements.

While ergonomics considers users’ physiological interface with interactive systems, cognitive de- sign focuses on the psychological interface between users and computers. This will be addressed in the next section.

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