A chair should be topographically neutral. It should conform equally well to all body shapes, sizes, and contours without applying circulation-restricting pressure anywhere. While people of different body weights and builds distribute their weight on a chair in similar patterns, they are different when it comes to pressure intensity; this varies from person to person. The challenge is to engineer a chair so its structure and materials provide dynamic support. This allows the sitter’s body, rather than the chair’s structure, to dictate pressure distribution.

What We Know

Surface pressure can cause discomfort while sitting. People of different body weights and builds distribute their weight on a chair in similar patterns, but pressure intensity and areas of distribution vary from person to person. Good pressure distribution in a chair focuses peak pressure under the sitting bones in upright postures and in the lumbar and thoracic areas in reclined postures / See Figure 1 /.

Correct pressure distribution is critical to seated comfort (Grandjean et al., 1973). A high level of surface pressure can constrict blood vessels in underlying tissues, restricting blood flow, which the sitter experiences as discomfort.

                     / Figure 1 / The human spine and pelvis

To measure these small differences in pressure distribution and their relationship to chair comfort, researchers have experimented with a number of technologies. Most recently, thin, flexible, pressure-sensitive mats connected to computers have been used to “map” the pressure-distribution properties of seating elements in office, automotive, and medical applications. These sensor-lined mats are draped over the chair’s seat pan and backrest; when a test subject sits in the chair, pressure gradients show up as different colors on the computer screen, mapping the peak pressure zones experienced by the sitter (Reed and Grant, 1993).

Using pressure maps to evaluate chair design is not a straightforward process; different people sitting in the same chair may exhibit very different pressure maps, depending on their weight and build. For instance, while heavier people generally show higher pressure peaks than lighter people, a heavy, pear-shaped person may exhibit lower pressure peaks than a lighter person with less internal padding to sit on (Reed et al., 1994).

Because of the large variance in peak pressure patterns among people of different sizes and shapes, it is difficult to prescribe ideal seat and backrest contours or softness levels that would minimize uncomfortable pressure points for all sitters. We do know, however, that the skin and fat tissue under the ischial tuberosities, or “sitting bones,” are less sensitive to pressure than the muscle tissue surrounding the tuberosities and better suited to carrying load than the other tissues of the buttocks and thighs (Reed et al., 1994).

In addition, chairs with backrests that exhibit pressure peaks in areas of the lumbar away from the spine have been judged more comfortable than chairs that show lower pressure gradients in these regions (Kamijo et al., 1982), although pressures resulting from a very firm lumbar support can cause discomfort (Reed et al., 1991a, 1991b). Our own research has found a strong correlation (r=.638; n=978) between overall seated comfort and the degree to which the sitter perceives the chair as providing good lower back support.

As a sitter changes postures from upright to recline, however, pressure-distribution patterns change. How these patterns change is also a function of the chair’s kinematics—the mechanics of how a body moves through space and the range of postures that the chair supports.

Left: / Figure 2 / Good pressure distribution in a chair focuses peak pressure under the sitting bones in upright postures and in the lumbar and thoracic areas in reclined postures.
Right: / Figure 3 / Sitting in a sling-type chair puts pressure on the gluteus maximus muscles at the sides of the buttocks as well as on the heads of the femur bones and sciatic nerves.

Pressure mapping shows how seated body pressure is distributed. Red indicates peak pressure areas;
orange, yellow, green, blue, and purple indicate decreasing pressure areas.

Therefore

A comfortable chair will produce pressure distributions for a wide range of users that show peaks in the area of the ischial tuberosities when the sitter is in an upright posture and areas of the back away from the spine when the sitter is in a reclining posture / See Figure 2 /.

Design Problem

Design a chair that is topographically neutral, so that the sitter’s body, and not the underlying structures of the seat pan and backrest, determines peak pressure areas.

Chair designers try to minimize circulation-restricting pressure with the right combination of contour and padding, curving the chair’s structure away from pressure-sensitive areas of the body and cushioning it with foam. This is difficult to achieve in a design that must serve a diverse user population. Seat shapes that work well for the bone structure and leg length of a tall male are likely to hit a short female in all the wrong places. Foam density that provides optimal comfort for a small, plump woman may “bottom-out” under a heavier but leaner man.

Extra padding does not necessarily solve the problem, because a too-soft seat can put pressure on the gluteus maximus muscles at the sides of the buttocks as well as on the heads of the femur bones and the sciatic nerves, resulting in the kind of discomfort experienced when sitting in a sling-type playground swing or a director’s chair / See Figure 3 /. (Zacharkow, 1988; Hertzberg, 1958).

Design Solutions

Engineer the chair so the structure and materials provide dynamic support for the sitter and fit differently proportioned persons.

Instead of foam cushions that may impose improper contours, a work chair with a topographically neutral suspension will conform to the shape of the person who sits in it. Using pressure-mapping technology, we experimented with different tensions across the backrests and seats, fine-tuning our designs to produce the desirable distribution patterns: peak pressure zones under the ischia, with wide distribution of lower values along the thighs and across the back, avoiding the spine and the area behind the knees.

We were particularly interested in achieving a wide distribution of pressure across the backrest. While the seat of a chair typically carries most of the body’s weight, the more one reclines, the more weight is transferred to the backrest. We also know that a chair’s tilt range and kinematics—and the extent to which they mimic the body’s natural pivot points and more naturally transfer weight from the seat to the backrest—will encourage or discourage reclining. So the backrest may be called on to support a higher percentage of the sitter’s body weight. During development of our work chairs, we tested subjects of varying heights, weights, and critical body dimensions in different chair prototypes, controlling seat height and back-angle reclinations. Experimenting also with varying levels of perforation and tension of the suspension material, we worked to achieve a pressure-distribution pattern for a variety of body types across the sitter’s back, distributing weight away from the spine.

A topographically neutral seat and backrest, when designed to support a wide range of weights and proportions, ensured that people would get the benefits of the chair’s carefully tuned pressure distribution. Positioned comfortably on a topographically neutral suspension material, the sitter’s body, rather than the chair’s structure, dictates pressure distribution.

Pressure mapping technologies that measure the distribution of pressure across a chair’s backrest and seat pan have been refined since they were first used in the development of the Aeron chair. They now provide more detailed and accurate readings of the levels of pressure experienced by the sitter.

Although early pressure-distribution maps were made with subjects always sitting in reclined postures, we now know that small changes in the tilt of the backrest can result in large differences in the way pressure is distributed across the sitter’s back (Aissaoui et al., 2001). Using more advanced technology, we are now able to map and compare pressure distribution patterns in upright as well as reclined positions.

Understanding comfort and pressure distribution for sitters in upright postures has become more critical as a growing percentage of office tasks are accomplished using computer technology. Sitting behaviors research conducted by Herman Miller indicates that people performing computer-related tasks spend a greater percentage of their time in upright rather than in reclined postures (Dowell et al., 2001). Our understanding of optimal distribution patterns for people sitting in an upright posture has prompted us to look for particular patterns of pressure distribution depending on the sitter’s posture.

Pressure map studies of sitters in upright positions show bands of localized pressure where the lower back comes into contact with the chair’s lumbar support, but little pressure distribution across the rest of the back / See Figure 4 /. This stands in sharp contrast to pressure maps of sitters in reclined postures, which show distributed pressures in the thoracic area near the scapula and away from the spine / See Figure 2 /.

Hypothesizing that improved back support for upright postures would produce pressure distribution that more closely resembles that of reclining postures, we did pressure map studies of people sitting in topographically neutral suspension chairs with postural support. Chairs with sacral-pelvic support, designed to stabilize the pelvis, help to maintain natural spinal curvatures without applying pressure to the lumbar area.

Left: / Figure 4 / Sitting in an upright position in a chair with lumbar support shows bands of pressure
where the lower back comes in contact with the lumbar support.
Middle: / Figure 5 / Sitting in an upright position in a chair with postural support distributes pressure
across the sacral-pelvic, lumbar, and thoracic areas.
Right: / Figure 6 / Sitting in an upright position in a chair without postural support limits the distribution
of pressure across the sacral-pelvic, lumbar, and thoracic areas.

Maps of chairs with postural support show that pressure is distributed over a greater area, including the sacral-pelvic and thoracic as well as lumbar regions of the back / See Figure 5 / than in chairs without posture support / See Figure 6 / or those with lumbar support only. Subsequent research investigated the possibilities for a “load-leveling” design for both a chair’s seat and backrest. This work resulted in the development of the Pixelated Support system. In it, each component, or pixel, inherently conforms to the sitter’s weight, distributing the mass evenly on the seat and backrest. As a result, sitters experience a sensory experience of flotation.

To test the combination of the Pixelated Support system and the kinematics used in the Embody chair, we commissioned a study by the ergonomics laboratory at a leading university. Subjects sat in an Embody chair and four other chairs with various backrest designs and performed four tasks in a random order. Pressure-sensitive mats draped across the chairs’ backrests recorded the dynamic distribution and intensity of pressure throughout the trials.

As subjects’ torso angles became more reclined, the center of pressure shifted vertically regardless of chair. The increase in the average contact area, from upright keying to reclined video watching, was greatest for the Embody chair. This suggests that, when reclining, the Embody backrest provides greater support to the sitter’s upper back compared to other chairs (University of California Berkeley, 2008).

The shift in the center of pressure that varies from person to person is certainly one consideration for work chair design. Another is the challenge of designing a topographically neutral chair. It conforms equally well to all body shapes, sizes, and contours. Yet, it doesn’t apply circulation-restricting pressure anywhere. Reaching this goal requires successive advances in structure and materials that provide dynamic support. As these advances are commercialized, they bring us closer to the ideal—when the sitter’s body, not the chair—dictates pressure distribution.