Posture, Comfort, and Monitor Placement

[Citation: Ankrum, D.R. & Nemeth, K.J., (1995). Posture, Comfort, and Monitor Placement, Ergonomics in Design, (April. pp. 7-9.]

By Dennis R. Ankrum & Kristie J. Nemeth, Reprinted from Ergonomics in Design, April 1995, pages 7-9. Copyright 1995 by the Human Factors and Ergonomics Society, P.O. Box 1369, Santa Monica, CA 90406-1369, 310/394-1811, Fax 310/394-2410. All rights reserved.

Many VDT guidelines (for example, OSHA, 1991) have recommended placing the top of the monitor at or slightly below eye level. The reasons given for those recommendations include minimizing the postural load on the muscles of the neck (Canadian Standards Association, 1989) and preventing the user from tilting the head too far back or forward (Curtis, 1993).

Although well-intentioned in their attempts to encourage a natural and comfortable posture, such recommendations can actually produce opposite results. Maintaining any posture for long periods, no matter how mechanically efficient that posture may be, can be be unhealthy (Grieco, 1986). In this article, we propose an alternative to the eye-level or slightly below eye-level monitor position.

Posture and Vision Interact

The location of the visual target limits the range of postures an observer can comfortably assume. Sometimes viewers must strike a balance between postural and visual discomfort. For instance, when a visual target is to the extreme left or right side, the observer may twist his or her neck or trunk into an uncomfortable position in order to achieve visual comfort. Generally, head movement occurs with horizontal eye excursions of greater than 6 to 8 degrees (Fischer, 1925, cited in Von Noorden, 1985).

Users also have preferences for the vertical location of visual targets. Hill and Kroemer (1986) found that when users in an "upright" seated posture were shown targets 50 and 100 cm away, they preferred to gaze an average of 29 degrees below the Frankfurt Line (40 degrees below the Ear-Eye Line; see "Gaze Angle and Posture" on page 8). The closer the visual target, the lower the preferred gaze angle.

The preference for downward gaze angles for near work is consistent with the capabilities of the visual system. As gaze angle tilts downward, the resting point of vergence moves inward (Heuer and Owens, 1989). That reduces stress on the extraocular muscles. The ability to accommodate improves (Ripple, 1952), and reports of headaches (Tyrrell and Leibowitz, 1990), eye strain (Tyrrell and Leibowitz, 1990), and fatigue decrease (Owens and Wolf-Kelly, 1987). Because less of the cornea is exposed to the atmosphere, the risk of dry eye syndrome is reduced (Tsubota and Nakamori, 1993).

The eye-level monitor position is often determined with the user sitting tall (see "Gaze Angle and Posture"). In so-called normal, upright sitting (without a visual target), Hsiao and Keyserling (1991) found that subjects tilted their head and neck and average of 13 degrees forward from the upright position. With the monitor set to eye level, the user is presented with a choice: either assume a more erect head/neck posture than preferred or employ an upward gaze angle relative to the head.

Alternative Neck Postures

With any trunk posture users assume while using an eye-level monitor, when the head-erect posture becomes tiring, they have limited possibilities for relief. One option is to tilt the head backward (extension). Tilting the head back (without moving the monitor) lowers the gaze angle (relative to the Ear-Eye Line). Although this movement can benefit the visual system, bending the neck backward greatly increases postural discomfort.

In a study that simulated bifocal use, Kumar (1994) found that electromyographic (EMG) activity of the trapezius and sternomastoid muscles increased as neck extension increased. He also found greater discomfort as neck extension increased.

Another posture option with an eye-level monitor is the forward head position. In that position, the head remains erect while jutting forward from the trunk. Users sometimes adopt a forward head posture in a counterproductive attempt to relieve muscle tension caused by contracted neck muscles (Mackinnon and Novak, 1994). Forward head postures have been associated with cervical headaches (Watson and Trott, 1993), increased fatigue (Urbanowicz, 1991), and thoracic outlet syndrome (Mackinnon and Novak, 1994).

The remaining neck posture available is flexion, or forward bending. Chaffin (1973) found that 15 degrees of sustained neck flexion for a long period (six hours with 10-minute breaks each hour) resulted in no elevated EMG readings or subjective reports of discomfort. Sustained neck tilts of more than 30 degrees, however, greatly increased neck extensor fatigue rates.

However, when both flexing the neck and raising the eyes to look up at monitor, users experience significant visual discomfort. Each degree of flexion requires one degree of elevation in the line of sight (relative to the Ear-Eye Line). Menozzi et al. (1992) found that users experienced greater discomfort when looking up at close objects (relative to 11 degrees below the Ear-Eye Line) than when looking at objects at an equal but downward angle.

With an eye-level monitor and an upright backrest, computer users have been known to adopt a variety of awkward postures when moving from an upright, 90-degree posture. For example, they sometimes round their shoulders and upper back and slump forward (thoracic kyphosis). Looking up at the monitor from that posture requires extreme neck extension. Even worse is when a user leans forward to rest an elbow on the work surface and props chin in hand.

Lower the Monitor

Lowering the monitor increases the range of neck postures that a computer user can assume with a minimum of discomfort to either the visual or postural system. With a low monitor, the user can hold his or her head erect and direct the eyes downward. When the neck position becomes uncomfortable - as eventually any neck position will - temporary changes in the degree of neck flexion can provide momentary relief from discomfort with little or no cost to the visual system.

The Importance of Movement

Movement is critical to comfort at VDT workstations because it reduces static loads and increases blood circulation. There is a difference between required movement and voluntary movement. Required movements, such as head shifting between screen and document, are task dependent. Voluntary movements are postural changes that are allowed but now required by the task. We want to minimize the first and encourage the latter. For any trunk posture, monitor placement should allow for the widest range of neck postures with minimal posture and visual discomfort.

Preferred Gaze Angles

Reported preferred gaze angles, measured from the horizontal with unspecified posture, vary from the - 9 degrees at computer workstations (Grandjean, 1983) to - 38 for reading (Lehmann and Stier, 1961). Grandjean explained the difference by saying that "VDT operators definitely prefer a smaller viewing angle than readers" (Grandjean, 1983). However, factors other than the difference between VDT viewing and reading could explain the variance.

Glare was not controlled in Grandjean's study. In fact, one of the benefits found with the adjustable workstation (which included adjustable monitor tilt) was a lower incidence of "strong" annoying reflections compared with the old workstations. Even at the preferred screen inclination, however, 12% of the subjects reported strong annoying reflections. It is entirely possible that glare avoidance influenced the monitor placement preferences found in Grandjean's study.

Although it is recommended that the monitor be tipped backward so that the top of the screen is not closer to the eyes than the bottom (Ankrum, Hansen and Nemeth, in press), in many offices, doing this would result in unacceptable glare from ceiling luminaires. If glare and reflections are not satisfactorily addressed, new problems may be introduced. It is inappropriate to consider the monitor tilt adjustments and gaze angles commonly observed in office environments as indications of preferred settings, given the constraints of equipment and lighting systems.

A monitor placed at least 15 degrees below horizontal eye level would allow downward gaze angles when the head is erect, or in the more so-called natural position of 13 degrees forward head/neck tilt found by Hsiao and Keyserling (1991). Low monitor positions allow the user to alternate among a wider range of comfortable head and neck postures without sacrificing visual comfort. The common practice of placing the top of the monitor at eye level or slightly below eye level can constrain neck posture and should be avoided.

Gaze Angle and Posture

Gaze angle is the angle formed by a line from the fovea of the eye to the object of view and a second reference line, typically the horizon.

When describing postures, the terms upright, erect, and normal have not been clearly defined in the literature. As a result, reported measurements of gaze angles using the horizon as the reference line - even when a posture is indicated - are difficult to interpret.

Because visual characteristics such as the near points of accommodation and vergence change with movement of the eyes relative to the head, it is more appropriate to measure gaze angle against an internal reference point that moves with the head. A commonly used reference is the Frankfurt Line, a line that passes through the right ear hole and lowest part of the right eye socket.

The Frankfurt Line is generally difficult to identify without x-rays or palpation. A much more easily identified reference line is the Ear-Eye Line (Kroemer, 1994). The Ear-Eye Line (see illustration) passes through the ear hole and the outer canthus (the outer junction of the upper and lower eyelids). It is approximately 11 degrees inclined from the Frankfurt Line (Jampel and Shi, 1992). When the head tips, the Ear-Eye Line tips with it.

It is important not to confuse the Ear-Eye Line with the horizontal. In a self-defined "erect head position", Jampel and Shi (1992) found the Ear-Eye Line to be 15 degrees inclined from the horizontal.

Another important consideration is how changes in trunk posture may affect the orientation of the head and neck. Positioning the monitor at eye level with the user "sitting tall" makes the questionable assumption that the reference posture often depicted in engineering drawings (with the trunk and neck vertical and major joints at 90-degree angles) represents ideal practice. In fact, alternatives to the upright posture have been recommended - forward tilt (Mandal, 1981) and backward tilt (Grandjean, 1983) - and it has been noted that, if left alone, hardly anyone chooses to "sit straight" (Kroemer, 1988).

When measuring gaze angles to determine monitor positions, care must be taken to ensure that the user's postures are representatives of those actually used.

References

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Dennis R. Ankrum is Director of Human Factors Research at Nova Solutions, Inc. 421 W. Industrial Avenue, Effingham, IL 62401. He is exploring the interaction between vision and posture. Kristie J. Nemeth is a doctoral student in psychology and ergonomics at the Miami University Center for Ergonomic Research. She is studying work space design and working postures.