What We Know

Despite the fact that all human spines contain the same set of parts, the sizes and relationships among those parts vary. Over the years, we’ve discovered much about the differences and nuances of human spines.

Herman Miller was one of 40 industry partners participating in the CAESAR (Civilian American and European Surface Anthropometry Resource) survey, the first large-scale, three-dimensional anthropometric survey of civilians in the United States., the Netherlands, and Italy (the latter two are the anthropometrically largest and smallest populations in Europe). Prior to the CAESAR survey, anthropometric data was often based exclusively on a fairly homogenous test group—military personnel—and provided linear measurements only, such as lengths and circumferences. Because of the measurement techniques and population of subjects, the CAESAR survey results present a more comprehensive and realistic database of anthropometric information than previous surveys.

Through the CAESAR data, we have gained valuable knowledge about the diversity in shape and size of the civilian population. Moreover, CAESAR has provided us information about differences in body proportions, including the spine, and not just differences in weights and heights.

A brief anatomy lesson sheds light on the ways the Mirra 2 chair addresses spinal variations. / See Figure 1 /

The cervical region is in the neck area of the spine, from the skull to the shoulders. It is highly flexible and strong. The neck balances and supports the head, which weighs about eight pounds (3.63 kilograms). The thoracic region is the upper and middle parts of the spine. Because it is connected to the rib cage, the thoracic region has very little mobility. The lumbar is the region of the lower back that contains only five lumbar vertebrae, though they are the largest vertebrae in the spinal column. The lumbar region is capable of a great deal of movement. Below the lumbar area lies the sacrum, made up of five fused vertebrae held between the pelvic bones on each side.

When properly aligned and balanced, the thoracic spine region has a kyphotic curve, which is slightly convex, or outward. The cervical and lumbar spine regions have a lordotic curve, which is slightly concave or inward. Together, these 12 outwardly curved vertebrae and 12 inwardly curved vertebrae give the spine its “S” shape, essential to a healthy and strong back. They create the balance and the shock absorbers for the movements of the body.

These natural curves need to be supported and maintained to keep the back and spine relaxed and free of pain and pressure. They must also be supported as they change shape when the body moves. When a person moves from standing to sitting, the top of the pelvis tends to rotate backward, and the lumbar curve flattens or even moves into a kyphotic curve. Further, the pelvis dictates the curve of the spine in all seated postures. When the pelvis rotates out of its natural forward curve into a rearward position, pressure increases on the intervetrabral discs. / See Figure 2 / Also, muscle activity increases as the body attempts to restore balance lost when the lordotic curve changes. The result can be muscle fatigue and discomfort through the entire back; however, when the pelvis maintains a controlled forward rotation, the spinal curve of the back remains aligned both naturally and comfortably.

Historically, it was believed there was a relationship between a person’s thoracic and lumbar regions based on height and gender, but this has not been proven. The apex to the thoracic curve has a big variance. There is no relationship between the apex of the kyphotic curve of the upper back and the apex of the lumbar curve of the lower back. Through CAESAR, we learned we couldn’t be prescriptive; there was no perfect ratio to design to.

Through our ongoing research and participation in CAESAR, we understand the human anatomy in general and the seated spine and back in particular. Our understanding gives us the ability to design and produce products such as the Mirra 2 Chair that truly support the unique requirements of the regions of the human spine.

Therefore

A good work chair will address those unique requirements of the four regions of the spine: cervical, thoracic, lumbar, and sacrum. It will naturally support all the regions of the spine as a person shifts seated postures throughout the day.

And, a good work chair needs to be flexible in order to address the macro variables of human size and micro variables of back/spine scale.

Design Problem

The body needs to move. It’s a natural response, even when sitting. A Herman Miller study on seating behavior found that seated people move their torso an average of 53 times an hour. Nearly 28 percent of those moves involve leaning or turning (Dowell et al., 2001).

Most work chairs have backrests that are too rigid to allow the sitter’s torso to move the way it prefers. To compensate, sitters tend to move away from the backrest, thereby losing the support for the back.

Nor do most work chairs equitably support the entire spine, from the thoracic region to the sacrum, and the unique requirements of each of those spinal areas. While they may provide support to the lower back, the upper back is often overlooked.

With most traditional foam-and-fabric chairs, or those with integral frame structures, the contour of the backrest may mimic the general contour of the human spine, but it does not respond to the individual’s shape and scale or allow much flexibility in movement.

Design Solution

Provide a work chair that supports the entire spine and addresses the differences in the spinal anatomy within that support. The construction of the Mirra 2 Butterfly Back allows it to mirror naturally the leaning and turning motions of the sitter.

The Butterfly Back is supported by the Loop Spine, external to the chair’s backrest. It connects to the chair in only two areas—at the base and top of the backrest—which allows torsional flex. This design provides the sitter with the freedom to move, lean, and bend, while the Butterfly Back moves and bends in the same way. Without an integral structure, the Butterfly Back can flex as the sitter flexes, providing dynamic and responsive support and instant conformation to the sitter’s individual anatomy. / See Figure 3 & 4 /

/ Figure 4 / The Loop Spine connects to the chair in only two areas—at the base and at the top of the backrest—which allows flexibility. Without an integral structure, Mirra 2’s Butterfly Back can flex as the sitter flexes.

The material and design of the Butterfly Back is calibrated and tuned to the spine’s natural range of motion. The Butterfly Back’s gradient perforations resulted from engineering intentionally focused on the degree and direction of ergonomically-appropriate flex. The pattern allows for greater or lesser flex within the backrest, similar to the differences in the flexibility range of the spine itself. We refer to these areas of the backrest as flex zones. / See Figure 5 /

/ Figure 5 / Gradient perforations in the Butterfly Back are calibrated and tuned to the spine’s natural range of motion. The pattern allows for greater or lesser flex within the chair’s backrest, similar to the differences in the flexibility range of the spine.