Adaptation and Sensitivity to Postural Change in Sitting

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Date: Winter 2000
From: Human Factors(Vol. 42, Issue 4)
Publisher: Sage Publications, Inc.
Document Type: Article
Length: 5,712 words

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We used 3 psychophysics methods to determine perceptible changes in seat height, seat pan angle, and backrest angle using an experimental chair. In the method of adjustment, the chosen chair settings were affected by the initial setting. For example, a high initial setting of the seat height led to a high selected setting and a low setting led to a low value. The difference between settings was referred to as not noticeable difference (NND). The method of limits was used to determine acceptable chair settings using verbal limits such as "too high" and "too low." Using the method of constant stimuli, just noticeable differences (JNDs) were determined for chair height (1.5 cm), seat pan angle (1.2[degrees]) and backrest angle (1.7[degrees]). The corresponding values for NNDs and verbal limits were about twice as large: chair height (2.5 cm), seat pan angle (4[degrees]) and backrest angle (3[degrees]). NNDs and verbal limits are unobtrusive measures that are considered more valid than JNDs, which exaggerate the need for adjustability. The results have practical implications for the design of office chairs.

INTRODUCTION

Adjustable office chairs are often not adjusted. Many reasons for this have been provided in the literature: Users are unaware of adjustability features (Helander, Zhang, & Michel, 1995; Lueder, 1994), they do not understand how the mechanisms operate (Kroemer, 1997; Lueder, 1994), users need training (Cornell & Kokot, 1994; Dainoff, Mark, Moritz, & Vogele, 1987), and some users consider adjustability unimportant. This study deals with the perceptibility of changes in chair settings. The premise is that in order to adjust a chair, users must at least be able to sense differences in chair settings. If users can discriminate among only a few different positions, adjustability can be dealt with using simple mechanisms that can be set in only a few positions.

Previous Research

Several studies have reported preferred postures in VDT work. Typically the preferred seat height, seat pan angle, backrest angle, and table height are recorded at workstations in offices (e.g., Grandjean, 1987; Grandjean, Hunting, & Pidermann, 1983; Lueder, 1994; Rinalducci, 1983).

However, psychophysics approaches to measurement of adjustability features are rare; we found only one relevant study in the literature. Well before the days of adjustable office furniture, Le Carpentier (1969) performed a study on preferred settings for easy chairs. In his study, participants adjusted an easy chair to the preferred positions for seat height, seat pan angle, and backrest angle. Participants were instructed to assume that they were reading or watching television. The method of limits (discussed in his paper as method of adjustment) was then used to derive limits for just noticeably less comfort and decidedly less comfort as compared with the preferred position. The experimental design was similar to the method of constant stimuli, with incremental changes in up/down settings. Because his study employed a different methodology than the current study, it is difficult to compare the two.

Habituation errors and adaptation errors. Errors arising from habituation are well documented in the psychophysics literature (e.g., Gescheider, 1997). An...

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Gale Document Number: GALE|A74523964