Sauter (1986) found that out of 40 VDT operators typing at a work surface fixed at the standard typing height (approximately 26 inches) over 75 percent of them sat significantly higher than their popliteal height. Of these operators, there was no apparent statistical relationship between leg and thigh discomfort and seat height. (Note: in these examples, although subjects sit higher, their feet are still supported by the floor.)
Where is the evidence for the 90° knee angle? I haven’t seen it, and suspect it doesn’t exist. Near as I can figure, it is based on the recommendations of Akerblom (1954), although prior recommendations of a similar nature were made by Staffel (1884) and Hooton (1945). It was supposed that this posture would help to prevent leg swelling. Since that time, these recommendations seem to have been accepted without question.
Leg edema is a genuine problem. Widmer (1978) interviewed 4,529 workers, and found that that 44 percent of men and 70 percent of women experienced discomfort from leg swelling. It is also problematic because local pooling of the blood increases the hydrostatic (venous) pressures to the heart, and thereby increases the blood pressure and heart rate. It likely predisposes the users to a number of peripheral venous disorders such as varicose veins and reduces the temperature of the lower legs, a strong predictor of cold discomfort.
However, leg edema appears to be a function of being sedentary, rather than excessive seat heights. During movement, the associated muscle groups extend and contract, and thereby promote circulation. Pottier, Dubreuil, and Monod (1969) placed wood slats 8 cm high at the front, at the front of the thighs of a group of 32 subjects and found that, even in this worst case condition, a maximum of 25 percent of the lower leg edema could be attributed to thigh compression. Winkel (1981) found that when subjects performed intermittent exercise, most of their leg swelling disappeared within a short time. Winkel and Jorgensen (1986) found that during inactive sitting foot swelling averaged 4.8 percent and muscle temperature of the lower legs (hallucis longus) decreased considerably, but when the seated subjects moved about at regular intervals, leg swelling only reached .8 percent and muscle temperature remained virtually unchanged.
Other solutions to alleviate this problem include pulsating (Circutone) seat cushions that stimulate circulation (Shvartz, Gaume, White, and Reibold, 1980) and "dynamic" footrests that allow the user to exercise during work (Stranden, Aaras, Anderson, Myhre, and Martinsen, 1983).
However, leg swelling is not the only consideration in determining the optimum seat height. It is widely acknowledged that one of the primary problems of the seated posture is that the lower (lumbar) spine is excessively flattened. Bendix (1984) found that lumbar lordosis was improved when the seat height was increased, although these results were also related to the seat pan angle. Although there is no available research on the subject, I suspect that people like to sit higher than their popliteal height because it opens up the thigh-trunk angle and thereby alleviates stresses on the lumbar spine.
In the process of working on occasion with a consortium of colleges of chiropractic I have learned that there is a tremendous variability in the contours of the spine within the general population, even when torso length is accounted for. It seems plausible that the biomechanics of assuming the seated position is to some extent specific to the individual and the task. If so, we need to better formulate what the user needs to know to decide for themselves how to sit.
Nonetheless, it might be asked, in the absence of unequivocal information regarding the optimum seat pan height, what is the harm in recommending that the seat adjust to the popliteal height? Well, at least two reasons come to mind.
First, it is expensive to comply with this lower level both because of retooling costs and frequently from problems in maintaining the structural integrity of castored seats. Increasing the cost for the seat serves as a disincentive for client organizations to buy "ergonomic" seating.
Second, and more importantly, seat mechanisms typically afford a fixed range of adjustability. When the minimum level of seat height is lowered, the maximum is correspondingly reduced. As a result, a number of furniture manufacturers have independently volunteered to me that in order to comply with the minimum seat height stipulated in impending ANSI guidelines, they had to lop off the top end of the range of their seats.
There are three issues that should be considered when evaluating the adequacy of the upper range of seat pan adjustability. The most obvious one is that large users should be accommodated. Second, in order to benefit from the forward sloping seat pans recommended by Mandal (1984), they need to adjust considerably higher.
Third, users consistently adjust their seat height to allow a comfortable working arm posture. As a result, when the work surface height is fixed, small females sit higher than large males (Burandt and Grandjean,1963; Floyd and Roberts, 1958; Floyd and Ward, 1964; Langdon, 1965). Langdon (19.65, p.66) notes "in consequence, although the chair is raised to the point where the limbs cannot be accommodated beneath the desk and the feet cannot reach the floor, the operator is well above the keyboard and sits on the edge of the chair, gripping the curved feet of the chair frame with her shoes".
Ironically, pending seat height recommendations that attempt to address the difficulties of the small female inadvertently increase the potential for the worst case situation, in which her seat is fully jacked up, legs unsupported, yet she must still maintain excessively elevated arms.
Hopefully, in the coming years, mechanisms will be developed and used in seating that provides greater ranges of adjustment. If so, these problems may be short term. However, the HFS community has placed such a great emphasis on the lower end of the spectrum that the importance of the upper range of Seat heights is not well understood by industry. (Sidebar:This article was written in 1986, before the extended cylinders came out. The current research on the topic continues to support the main points of this article, however.)
It might be argued that much of the problem is really a function of work surfaces that are typically fixed at 29 inches; I could not agree more. In order to accommodate a spectrum of user sizes and seat pan angles, non-adjustable work surfaces are becoming increasingly problematic, particularly when postures are constrained such as during VDT operation.
It may also be argued that existing recommendations for popliteal heights are primarily based on the U.S. military population, and that we should consider the fact that immigrants may have popliteal heights as low as 35.5 cm (14 inches) at the fifth percentile (Webb Associates, 1978); I have no answer to this problem.
But I don’t think that the prevailing directives address the underlying problems either. My feeling is that we need to fundamentally rethink our recommendations and conduct more research that asks intelligent questions before we should feel confident in telling manufacturers how to design their seats and users how to sit.
Rani Lueder has consulted in occupational ergonomics and product design research since 1982. Her activities in sitting and seating include co-organizing the Second International Symposium on Seating, held in Tokyo. Her second edited book, on sitting postures (Taylor & Francis), is sold worldwide. Since 1988, she continues to consult on sitting posture on extended retainer for a range of organizations in Japan. She served on the seating subcommittee for the American National Standard Institute guideline ANSI HFES 100-2007. She has consulted in the design of more than 250 lines of seat design and other products that support posture. She edited the book Ergonomics for Children; Designing products & places for toddlers to teens (2008, Taylor & Francis)
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