Chairs for Pain-Free Computer Work
A Chair Must Closely Fit the Sitter to Protect Spine and Muscles
Jul 11, 2008
There are two principles of ergonomics (the science of how humans interact with their surroundings) a chair must meet to protect against the strain and injury caused by sitting for long periods: it must fit the sitter and allow movement. This aids productivity since, without comfort, a person is likely to get up and leave the desk often.
The chair's backrest must be sized and shaped to support the natural curvature of the spine. Orthopedic specialist Doctor Mark Vettraino, Director of Task Group International, states that if the curvature is distorted by leaning forward, the disks can bulge and painfully abrade nerves, triggering muscle spasms.
Correct size and shape of the seat reduces pressure on the thighs and legs that can restrict blood flow.
Some office chairs allow adjustments to be made to match the sitter's size and weight, as explained in another article. But the chair must closely fit to start off with.
Back Health
Many brands of office chairs are made in three sizes, each available with three backrest heights: up to just below the shoulder blades; up to shoulder height; and head height.
In many chairs, the backrest is a hard plastic or wood shell covered with upholstered foam. In others, a rigid frame supports a resilient mesh that allows air flow around the sitter's back for cooling. In both constructions, the backrest is shaped to conform approximately to the sitter's back.
For best comfort, the backrest should support the upper as well as the lower back, and curve around the body to support the upper back, allowing muscles to relax and aiding the sitter's breathing.
The lower section of the back rest should cause the pelvis to tilt forward slightly. In most modern office chairs, the backrest is separate from the seat, making a space for the curve at the base of the spine.
Comfortable Seat
The seat should have a "waterfall" front; i.e., the edge curves downwards. This reduces pressure under the thigh. The sides should curve up slightly to support the outer edge of the hips and distribute weight away from the seat bones. The seat should be about 2 inches or 5 centimetres wider than the sitter's hips and thighs.
To prevent pressure on the legs that can restrict blood flow, the seat should leave a gap equal to the width of three fingers between the seat edge and the back of the knees when the person's back is against the backrest.
In many chairs, the seat consists of a rigid shell covered by upholstered foam, which should maintain resiliency for several years. Professor Alan Hedge, director of Cornell University's Human Factors and Ergonomics Research Group, says a person should be able to sit in a chair for one to two hours without discomfort. Anything less than that and the density of the foam is too low or it has lost resiliency.
Some mesh-backrest chairs use mesh on a frame for the seat also. Some manufacturers combine a mesh backrest with a foam seat because they believe the foam gives a better waterfall front.
Relieve Stiffness
The ability of the chair to allow the sitter to move and rock is important to relieve muscles and joints of stiffness, promote blood flow and allow disks to flex. It should allow the sitter to move while seated without straining to reach items on the desk.
The size and shape of the chair, the chair's ability to provide fit and movement, are fundamental to health and productivity. Backrest and seat should closely match the sitter's body and reduce pressure from the seat. The cushioning of the foam or mesh should last for years, during which it should allow a person to sit comfortably for one to two hours at a time.
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