Cornell University Ergonomics Web

DEA 3250/6510 CLASS NOTES

Sitting and Chair Design

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. How we sit and what we sit on affects the health of the spine. The lumbar region is the most frequently damaged (L4 and L5). The vertebrae in the lumbar region are the largest in the spine.

• Definition - Sitting is a body position in which the weight of the body is transferred to a supporting area mainly by the ischial tuberosities of the pelvis and their surrounding soft tissue.
• Purpose - to remove weight from the feet and maintain a stable posture so muscles not directly involved with the work can relax.
• Ideal - There is no single ideal sitting posture. Illustrated 90-degree person sitting posture is for anthropometric reference only. Can't design a chair for the best single way to sit. We need a variety of chairs that allow different users to each sit in a variety of postures.

2. Posture - the relative orientation of parts of the body in space.

• Best Posture - imposes the least postural stress. Muscles must do work to counteract the effects of gravity and other forces as the body stands or moves through space.
• Postural Strain - adverse consequences of more than a few minutes of postural stress.
• Fidgeting - is the bodies defense against postural stress of which discomfort is a sign.
  • Rate of Fidgeting - can be used as an index of chair discomfort. Higher fidgeting rates correspond to higher discomfort rates.
  • Crossing and Uncrossing the legs - is a characteristic way of re-distributing pressure on the buttocks and also helps to pump blood through these tissues.
• Postural Comfort - is defined as the absence of postural discomfort, it is therefore a neutral state that we cannot sense.

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.

• Lumbar Region - is normally lordotic (concave, toward the stomach). This reduces the pressure between the vertebrae. The region is normally lordotic for two reasons:
  • Thickness - Vertebrae and discs are thicker anteriorly than posteriorly.
  • Sacrum - Upper surface of sacrum is at an angle to the horizontal plane.
• Pelvis - The sacrum is fixed to the pelvis, so rotational movement of the pelvis affects lumbar vertebrae.
  • Forward rotation - of the pelvis leads to increased lordosis of the lumbar spine, helping to maintain an upright trunk position.
  • Backward tilt - of the pelvis leads to increased flattening of the lumbar spine and eventually increases kyphosis.

4. Sitting Postures - Sitting with the knees and hips flexed, pelvis rotated backward leads to minimize lordosis, flat, or even kyphotic lumbar spine. Three types of sitting postures normally distinguished:

• Anterior (forward leaning) - center of mass in front of the ischial tuberosities. Floor supports more than 25% of body weight. Common posture for desk work.
• Middle (relaxed, unsupported) - center of mass directly above ischial tuberosities. Floor supports 25% of body weight. Straight or slightly kyphotic lumbar spine.
• Posterior (backward leaning) - center of mass behind ischial tuberosities. Floor supports less than 25% of body weight. Common for chairs with large, inclined, backrests. Preferred for resting.

5. Factors Influencing Posture -

• Hamstring muscles - also influence sitting posture and configuration of the lumbar spine. These muscles cross from lower leg to pelvis, crossing the hip and knee joints.
• Location and slope of work area - have a major influence on postures of neck, shoulders, and upper extremities and therefore also must be considered along with the design of the seat, and the sitting habits of the person.

6. Preferred Sitting Angle - Sitting leads to 40 - 90% more stress on the back (disc pressure) than standing posture.

• Studies - From a study of college students the preferred seat back angle for comfort is 15-degrees. This is in keeping with other studies by Etienne Grandjean where VDT operators have preferred 13-15-degree backward incline. At this angle the pressure on the intervertebral discs is minimal.
•  "Keegan's normal posture" - X-ray studies on 5 people lying on their sides (1955, 1960) show that stresses on the spine are most evenly distributed when legs are at 135-degrees from torso, i.e. 135-degrees hip angle and 45-degrees knee angle.

7. Seat Design Criteria

• Seat Height - Optimum seat height is controversial.
  • Traditional Criterion - Seat height should be adjusted to support a knee angle of 90-degrees to prevent leg swelling. However 75% of leg swelling may be due to low leg muscle activity rather than chair.
  • Minimum Height - should be 15" (38cm) which designs to the 5th percentile of women with 1" heels. The seat should adjust 9" (23cm).
  • Fixed Height - should be about 17" (43cm). This is a compromise. A chair that is too high leads to increased pressure at the popliteal fold (underside of knees), decreasing blood circulation and increasing pressure on the nerve. A chair that is too low increases weight on the ischial tuberosities.
• Seat Pan -
  • Seat Depth - recommended is 16.5" for fixed seats and 14-18.5" for adjustable seats. If the seat depth is greater than the buttock-popliteal length (fifth percentile woman is at 17") then the user won't be able to use the backrest.
  • Seat Pan Contours - Half body weight is supported by an 8% area under the "seat bones" (ischial tuberosities). If the seat is hard and flat the pressures can be 85-100 p.s.i. Seat contouring and cushioning can be used to distribute pressure over a larger area and rotate the pelvis forward the promote better posture.
  • Seat Cushioning - recommended thickness at 1.5-2". Cushion should be firmer in back and thicker while less firm and thinner at front. Too much cushioning can cause the body to sink into a chair constraining movement. A soft chair may be comfortable at first, but as the body sinks blood circulation lowers, skin temperature rises in affected areas, and compression under thighs increases. These factors combine to increase discomfort.
    • Cushion Compressibility - Compressibility is termed indentation load deflection (ILD) or indentation force deflections (IFD). An ideal combination is a soft top layer (25% ILD) over a firm bottom layer (65% ILD). Increased ratios between the two, greater than 2.6, leads to better quality support.
  • Seat Width - around 20 - 22" to accommodate clothed persons. If seat has armrests then elbow to elbow breadth may be more relevant.
  • Seat Angle - Positive seat angle helps user to maintain good contact with backrest. For most purposes a 5 - 10 angle is recommended.
• Armrests - give additional postural support and aid in standing up and sitting down. Armrests should be padded and engage the fleshy part of the forearm. They should not engage the bony parts of the elbow where sensitive ulnar nerve is close to the surface so a gap of approximately 4" between the armrest and seat back is recommended. Cantilevered elbow rests should be 8-10" above the seat surface height. Armrests should be at least 17.2" apart to exceed thigh breadth of 95th percentile females. Finally, armrests shouldn't limit chair access if it is to be used at a table.
• Backrests -
  • Height - Higher backrests give better trunk weight support. Three categories:
    • Low-level backrest - supports the lumbar region only. Depth of the lumbar curve of the backrest should be 0.6 - 2.0". Backrest heights of 5, 7, and 9" seem equally effective.
    • Medium-level backrest - gives full shoulder support (e.g. car seat, office chair) and may need to be about 26" high to accommodate the 95th percentile man.
    • High-level backrest - full support of head and neck (e.g. plane seat) and may need to be about 36" for a 95th percentile man.
• Angle - Optimal angle seems to be between 100-110-degrees.

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