1. Introduction - An estimated 50% of people in the industrialized world suffer some form of back complaint and many of these are related to poor seat design. sit and what we sit on affects the health of the spine. is the most frequently damaged (L4 and L5). The vertebrae in the lumbar region are the largest in the spine. 2. Posture - the relative orientation of parts of the body in space. 3. Biomechanics of Sitting - depending on chair and posture, some proportion of total body weight is transferred to the floor via the seat pan and feet, armrests, and backrests. 4. Sitting Postures - Sitting with the knees and hips flexed, pelvis rotated backward leads to minimize lordosis, flat, or even kyphotic lumbarThree types of sitting postures normally distinguished: 5. Factors Influencing Posture - 6. Preferred Sitting Angle - Sitting leads to 40 - 90% more stress on the back (disc pressure) than standing posture. 7. Seat Design Criteria
Back to DEA 3250/6510 menu The Kinematics of Sitting A chair should move the way the body moves. In the best of all possible worlds, the body is free to position itself spontaneously, constrained only by gravity. A person seated at work should be able to move freely and unselfconsciously from computer-related tasks to more relaxed or interactive postures. The work chair should follow along, providing optimal support whether the body is in motion or at rest. People assume many different positions when they sit at work. Movement while seated is healthy. People rarely adjust their chairs. People who sit at their work rarely sit still. Field studies of people working at VDTs (Grandjean et al., 1983; Grieco et al., 1986) found them assuming a wide variety of postures even while performing a single task. Our own research and observations of seated behavior in the office identified three distinct modes of sitting at work: Forward sitting: used for performing work on the plane of a desk or for interacting with office equipment (Mandal, 1985).
(People of small stature working at a fixed-height work surface are virtually forced to assume this posture.) Slightly reclined sitting: used for conversation, telephoning, keyboarding, and mousing. Research shows that it is a preferred work posture (Grandjean, 1980; Deeply reclined sitting: used for resting, reading, and, in some cases, keyboarding. Experts agree that changing positions at work has important benefits for the sitter: Muscle movement serves as a pump to improve blood circulation (Schoberth, 1978), movement of the spine nourishes the intervertebral discs (Holm and Nachemson, 1983), reclining while seated pumps nutrients to the discs (Andersson, 1981), and continuous movement of joints is therapeutic for joints and ligaments (Reinecke, 1994). But if a chair requires its user to adjust it in order to shift into another position, it may have the effect of keeping the sitter undesirably still. Studies of people sitting at work indicate that they tend not to use manual adjustments on their chairs (Kleeman and Prunier, 1980;
A good work chair allows a person to sit comfortably and properly supported in each of the three basic work postures and to move spontaneously between them while maintaining that comfort and support. Support the lower back’s natural curve in all the sitting postures assumed at work without requiring the sitter to make conscious, active adjustments to the chair. In the seated position, the body does not automatically adjust to achieve optimal spinal and pelvic alignment. The unsupported lower back tends either to straighten or to slump in an outward, kyphotic curve rather than the more healthful inward, lordotic curve it naturally assumes in a standing position (Andersson et al., 1979). Studies of the seated body have shown that the position of the pelvis determines the shape of the spine (Schoberth, 1970), due to the relatively rigid connection between the sacrum (the base of the spine) and the pelvis. In a seated position, the pelvis tends to rotate backward, causing the lumbar spine to flatten or curve outward (Andersson et al., 1979).
Providing support that stabilizes the pelvis to prevent backward rotation ensures a natural curve, whereas applying pressure to the lumbar spine does not (Kroemer, 1971; A chair that provides support that nests the sacral-pelvic area as well as a deliberately placed target to engage the ischial tuberosities (sitting bones) creates a pocket that holds the pelvis in its natural position. / See Figure 1 / / Figure 1 / Provide support that nests the sacral-pelvic area and tilt action that echoes body mechanics. The design of the Aeron chair bypasses current mechanical models in favor of a tilt geometry based on human body linkages. Bill Stumpf’s research with Roger Kaufman at George Washington University identified the relationship of the body’s major pivot points as it moves between the three basic seated postures. If it were possible for the body to move from an upright seated position to a reclined position without the support or constraints of a chair, this is what it would look like: / See Figure 2 /
/ Figure 2 / With the feet flat on the floor and a slightly open angle between the lower and upper leg, all of the body’s major joints pivot about the center of the ankle joint (A). The knee joint (B) actually doesn’t pivot so much as travel in a gentle arc. The hip joint (C) follows an inclined arched path about a line connected to the ankle pivot (A) and pivots as the trunk-to-thigh angle opens. The arms pivot about the shoulder joint (D). Throughout the movement the head pivots about the cervical spine (E) to maintain a constant relationship between the face plane and a visual target. The linkages of the Aeron chair were designed to echo these body linkages in a movement that corresponds (coheres) to the natural movement (kinematics) of the human body. We call it the kinematic coherence model and refer to the mechanism as the Kinemat tilt. In an upright position, the self-shaping ischial target in the seat pan and the contours of the backrest create a pocket to hold the pelvis at a slight forward tilt to enhance lordosis.
/ See Figure 3 / / Figure 3 / As the chair reclines, the feet remain flat on the floor as the lower leg pivots around a stationary ankle joint. The backrest drops about the hip pivot point, maintaining the same point of contact between the backrest and the pelvis throughout the range of movement. / See Figure 4 / / Figure 4 / The seat pan drops about a pivot point in the ankle joint in a synchronous relationship to the backrest to maintain the pelvic pocket and preclude lumbar shear (in which the chair’s lumbar support moves away from the sitter’s lumbar region) in all tilt angle positions. Armrests move with the backrest to support the arms as they drop back in their natural rotation at the shoulder joint. / See Figure 5 / / Figure 5 / The Aeron chair supports the body’s natural linkages at all points, in all positions. As the sitter moves from upright to reclined, the feet are not lifted from the floor, as in column-tilt chairs; the back support does not lose contact with the sitter’s back, as it does in many synchronous tilt chairs;