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 Ergonomic criteria for the design of a new work chair by Bill Stumpf, Don Chadwick, and Bill Dowell A chair should fit the body like a piece of clothing What we know People vary considerably in shape as well as overall size. In addition to the 17 inches in height and 140 pounds in weight that separates a 1st-percentile female from a 99th-percentile male (Gordon et al. 1988), there are gender-related differences in bone structure and weight distribution and infinite variations in limb lengths and body contours. Even among a group of people of the same gender, age, and stature, one finds significant variation in bodily proportions (Pheasant 1986). Two men of the same standing height, for instance, can appear to be of very different heights when seated, and their seated elbow heights may vary by as much as three or four inches. Achieving a match between certain body dimensions and corresponding chair dimensions is crucial to the sitter's comfort and health. The wrong seat height can cause uncomfortable pressure on the backs of the thighs (Bush 1969). A seat pan that is too wide or too deep may prevent the sitter from taking advantage of armrests and backrest contours that help to transfer weight from the spine (Occhipinti et al. 1985, Andersson et al. 1974). Most work chairs are designed on a "middle-out" model of anthropometrics intended to accommodate the middle 95 percent of the user population: from the 5th-percentile female to the 95th-percentile male. However, as British ergonomist Stephen Pheasant points out, there is no true 5th- or 95th-percentile person; someone who is at the 95th percentile for stature is likely to be at a different percentile on distribution curves for lower leg length or sitting elbow height. So a chair designed to accommodate the middle 95 percent on each of a succession of important dimensions could conceivably exclude a different 5 percent of users with each anthropometric constraint. The end result would be a chair that accommodates considerably less than 95 percent of its potential users. Compounding the problem is the fact that the anthropometric data used by chair designers do not necessarily reflect the total adult population that will be using their product. This makes it virtually impossible to determine the actual percentage of users that will be fit for any given dimension. Commonly accepted anthropometric tables are based on samples of military personnel which (due to entry and retention criteria for size, age, and physical condition) tend to exclude very large and very small persons. Analyzing our own random sample of the U.S. civilian population, we found that a chair designed for the 5th-percentile female to 95th-percentile male--as defined by standard anthropometric data published by the U.S. military (Gordon et al. 1988)--would actually fit slightly less than 68 percent of the sample, even when considering only the four most crucial seating dimensions. Using a measuring device we developed to gather our own anthropometric data, we took seven important measurements: - popliteal height (lower leg length)
 Therefore: To be truly supportive of a large percentage of the working population, a work chair must accommodate people outside the 5th to 95th percentiles on distribution curves for several relevant body dimensions. Design Problem: Design a chair that fits smaller and larger people as well as it fits "average" people. Most work chair designs try to accommodate people of different sizes and shapes with a series of mechanical adjustments. These adjustments have some obvious physical limitations. For example, while the variation in lower leg lengths of the adult American population spans more than six inches, chair height adjustment mechanisms generally are not engineered to provide more than four-and-one-half inches of adjustment. A chair that is designed to meet the needs of the hypothetical 50th-percentile person becomes less and less accommodating as it is adjusted toward the requirements of the (equally hypothetical) 5th-percentile female or 95th-percentile male. In addition, our own field observations indicate that the greater the range of adjustment provided, the greater the chance that a person will use the chair at an inappropriate setting. People are more likely to get proper support from a chair that requires only minor adjustments to fine-tune the fit. Design Solution: Provide the same chair in three sizes; make the seat and backrest of material that automatically accommodates differences in body shape. Concluding that no single chair could cost-effectively provide the necessary adjustment ranges to fit the 1st to 99th percentile for every important seated dimension, we designed the Aeron chair in different sizes, like a bicycle or a pair of shoes. Instead of following the traditional "middle-out" model, we took an "ends-to-the-middle" approach (see figure 2), designing the smallest chair for the smallest user, the largest chair for the largest user, and, finally, a midsized chair to cover the range not accommodated by these two. By designing first for the extremes, we developed a chair that gives virtually every person a reasonably good fit, even if it's never adjusted. The range of fine-tuning adjustment required for each chair becomes both easily manageable and mechanically feasible.
In comparison to a chair designed for the 5th-to-95th percentiles, a 1st-to-99th design fits a surprisingly greater percentage of an actual user population. Applied to our own previously cited sample of 778 U.S. civilians, a 1st-to-99th design fit 95 percent of the sample on all four crucial dimensions, compared to the slightly less than 68 percent that would have been fit by a 5th-to-95th design. Subsequent field studies using our measuring device examined the relationship between sizes of people and their preference for chair size (Dowell 1995b). Measurements of 224 people--in a sample that was evenly distributed between men and women and that closely reflected the distribution of the U.S. population on most dimensions--found that of all the anthropometric dimensions measured, height and weight had the strongest relationship to chair size preference. The relationship is strong enough to allow us to recommend a chair size based on those dimensions (see figure 4). Figure 4
Codesigner of two groundbreaking ergonomic work chairs for Herman Miller, Don Chadwickhas been instrumental in exploring and introducing new materials and production methods to office seating manufacture. His award-winning design for modular reception seating was introduced by Herman Miller in 1974. As research program manager for Herman Miller, Bill Dowell has studied anthropometry and pressure distribution and conducted field research on the components of subjective comfort. He is a member of the Human Factors and Ergonomic Society committee revising the ANSI/HFES VDT Workstation Standard.
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