Elsevier

Applied Ergonomics

Volume 32, Issue 4, August 2001, Pages 313-325
Applied Ergonomics

A field evaluation of monitor placement effects in VDT users

https://doi.org/10.1016/S0003-6870(01)00014-XGet rights and content

Abstract

Appropriate visual display terminal (VDT) location is a subject of ongoing debate. Generally, visual strain is associated with higher placement, and musculoskeletal strain is associated with lower placement. Seeking resolution of the debate, this paper provides a comparison of results from previous lab-based monitor placement studies to recommendations and outcomes from viewing preference and neutral posture studies. The paper then presents results from a field study that addressed two outstanding issues: Does monitor placement in a workplace elicit postures and discomfort responses similar to those seen in laboratory settings? Results showed placements in the workplace elicited postures similar to those in lab studies. Additionally, preferred VDT location generally corresponded to the location in which less neck discomfort was reported, though that trend requires further investigation. Overall, there seems to be consistent evidence to support mid-level or somewhat higher placement, as a rule-of-thumb, considering preferred gaze angle and musculoskeletal concerns. However, optimal placement may be lower for some individuals or tasks.

Introduction

Visual discomfort and musculoskeletal discomfort, particularly in the neck and shoulders, are occupational health concerns for people who work with computers (Bergqvist and Knave, 1994; Bergqvist et al., 1995; Hünting et al., 1981). Computer monitor placement is a key facet of workstation design. Although it is a factor that has been linked to worker discomfort, disagreement seems to persist regarding optimal placement. Standards only seem to rule out extreme locations, while more specific recommendations can conflict when based exclusively on concern for visual strain or musculoskeletal strain.

The USA standard for visual display terminal (VDT) workstations, which cites early studies of user preference (Miller and Suther, 1981; Grandjean et al., 1983; Weber et al., 1984), requires only that the entire monitor be placed within an envelope of 0 to −60° (that is, eye level to 60° below eye level; ANSI, 1988). ISO and Australian standards have a similar requirement (ISO, 1992; AS, 1990), although the latter also defines an optimum viewing zone from –15 to −45°.

Epidemiological research, lab-based experiments, and longer term field studies have been conducted to explore visual and musculoskeletal effects of monitor placement on computer users. In the aggregate, these studies seem to indicate some trade-off between visual strain and musculoskeletal strain with regards to optimal placement.

Higher monitor placement has been associated with visual stress (Bergqvist and Knave, 1994; 0 to −20°) and musculoskeletal stress (Bergqvist et al., 1995), though the latter is more often associated with lower monitor placement (Hünting et al., 1981; Grieco et al., 1982). At least two studies, however, have shown no link between musculoskeletal discomfort and monitor placement (Faucett and Rempel, 1994; Sauter et al., 1991).

Empirical placement recommendations range from 45° below the Frankfurt plane (Lie and Fostervold, 1994) to eye level (de Wall et al., 1992; Bauer and Wittig, 1998). In theory, the former reduces visual strain, the latter musculoskeletal strain. Such disparate recommendations may occur because studies generally do not investigate both areas of concern simultaneously. It follows that recommendations from such studies would minimize the targeted strain, even at the expense of the other.

Benefits to a lower gaze angle include reductions in ocular surface area (Sotoyama et al., 1996) and perceived viewing exertion (vergence effort) (Menozzi et al., 1994; Tyrrell and Leibowitz, 1990; Jaschinski et al., 1998). Results from several visual preference studies are summarized in Table 1. Except for the work by Hill and Kroemer (1986), mean preferences seem to be exhibited for gaze angles about 23–34° below the eye–ear line (GA-EEL), and centered about 9–10° below horizontal (GA-h). These studies tend to report wide ranges of results, reflecting extensive individual differences in visual capabilities.

Musculoskeletal strain can be objectively assessed through measurements of posture or muscle activity, and subjectively through reports of preference or discomfort. Table 2, Table 3 present results from several lab-based studies of the effects of monitor placement on those outcomes. These studies consistently show significant increases in activity of the cervical and thoracic extensor muscles with lower VDT placement (Table 2). Increased muscle activity is a concern, because localized muscle fatigue associated with continuous activation of low force/low threshold muscle fibers is thought to be a primary contributor to development of tension neck syndrome and myofascial pain in VDT workers and others who perform semi-static tasks (Hagberg et al., 1995; Sjøgaard and Søgaard, 1998). The studies in Table 2 found discomfort tended to be greater for lower placement, though not significantly so. Apparently many subjects do not experience discomfort while working at the computer, at least during the short exposures that are typical of lab studies. Subjects seemed to favor a mid-level location over lower locations. This is consistent with the visual field studies cited previously (Jaschinski et al., 1998; Grandjean et al., 1983), and the monitor location study by Burgess-Limerick et al. (1998). With the goal of reducing musculoskeletal strain, these results tend to support a mid-level or higher location, rather than placement below mid-level.

Table 3 shows consistency in induced postures among the studies (postures are depicted in Fig. 1). The data also show that there is some variability in the resultant postures, even for the same researchers working with different groups of subjects in different studies. Table 3 shows that mid-level placement consistently elicited gaze angles in the preferred range (GA-EEL in Table 1), while only about half the eye-level and moderate-low placements led to preferred GA-EEL. Mid-level placement also coincided with preferred GA-h. Evaluating the remaining data in Table 3 requires knowledge of neutral body postures, if it is assumed that strain expresses a difference from neutral.

A few studies have sought to define normal (neutral) head and neck postures, though Cailliet (1991) emphasized the distinction between correct posture and those that feel normal, but are structurally faulty; Gray et al. (1966) reported marked differences of 10–15° between “comfortable” and “best” measured postures. Nevertheless, some neutral head and neck posture data are presented in Table 4. Comparing that information with data in Table 3 suggests that neutral head posture is achieved at mid-level or moderate-low placements. Neutral neck postures seem to be attainable with all placements, but each placement can also result in pronounced neck flexion relative to vertical, that is relative to gravitational forces which the neck extensor muscles work against to maintain head and neck posture. The data in Table 3 suggest, then, that low placement would result in both visual and musculoskeletal strain, while the mid-level location seems to minimize postural and visual strain. Eye level and moderate-low locations each seem to provide some advantages and disadvantages.

What is lacking in this analysis is verification of the effects of monitor placement on posture in a workplace setting. An extended field study might also yield statistically significant information on visual and musculoskeletal discomfort, something that lab studies have not done. Additionally, such a study could provide preference data based on experience and context rather than first impressions. A field study was designed to address this void. Mid-level and eye level placements were studied, because those seemed likely to simultaneously induce less visual and musculoskeletal strain, based on the cited literature.

The primary questions of the study were:

Section snippets

Participants

Twenty software company employees volunteered for the study (8 women, 12 men; 11 software developers, 5 quality assurance analysts, 3 managers, and one technical support person). Averages for age, height, and weight were 36.6 yr (±8.9), 173.3 cm (±6.8), and 70.1 kg (±12.6), respectively. No participants wore bifocals. All looked primarily at the VDT during work, using few or no source documents. VDT use averaged at least 6 h/day. Monitors were either 43 or 48 cm in size (17 or 19 in, for 2 and 18

Results and discussion

The participants and conditions in which they normally worked were favorable for the purposes of this study. Participants were a homogenous group, all professionals, most with graduate degrees. They performed similar types of work and worked similar hours per week (35–40 h, considered full-time and average for the company). Rest break occurrence was found to be similar among participants, throughout the study. Results from the JCQ showed that participants experienced high job control and low

Conclusions

When considered jointly, this study and previous research in the areas of gaze angle preference and monitor placement support location of the center of the monitor at mid-level or somewhat above, rather than below, as a reasonable starting location, based on objective and subjective research outcomes. However, individual differences in visual capabilities (such as the use of bifocals), physical make-up, work tasks, and other workstation design elements signal that fine tuning of placement is

Acknowledgements

This research was supported, in part, by a gift from the Office Ergonomics Research Committee.

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