Office Chairs that Ease Back Pain
The Best Seating for Health and Comfort Can Be Adjusted to Fit
Jul 8, 2008
A chair designed to promote health and protect against physical stress or injury -- i.e., an ergonomic chair -- should meet two criteria ergonomists call fit and movement. To achieve this the size and shape should be a close fit (see Strain-Free Sitting at Computers). It should also be adjustable more than simply in height.
Back Support
The backrest should give support that maintains the natural curvature of the spine. Without this support, disks can bulge and irritate nerves and result in muscle spasms, according to orthopedic specialist Doctor Mark Vettraino, Director of Task Group International.
The backrest should be adjustable in height so its lower section causes the sitter’s pelvis to tilt forward slightly. Many ergonomic chairs have a component in the backrest foam or mesh that can be adjusted in height and forwards and backwards to adapt to the shape of the sitter’s lumbar spine.
If a headrest is attached, it should be adjusted to maintain curvature of the neck.
Healthy Seat
The seat pan should be adjustable forwards and backwards to fit thigh length. With the sitter’s back against the backrest, the distance between the front edge of the seat and the back of the knees should be about the width of three fingers to eliminate pressure behind the knees that can restrict blood-flow. A seat that can be tilted can further reduce pressure.
The seat height should be adjusted so the sitter’s feet are flat on the floor and the thighs are horizontal.
The height is controlled by a pneumatic cylinder and spring (a "gas lift"). Air in the cylinder supports and cushions the seat. When the sitter rises and operates a lever to open a valve, the spring raises the chair and a piston draws air in until the lever is released to close the valve. When the sitter operates the lever while seated, air is released to lower the seat.
Professor Alan Hedge, director of Cornell University’s Human Factors and Ergonomics Research Group, warns that some cheap chairs have inferior cylinders that can fail and cause injury.
Rest for Arms
Armrests should be adjustable in height. Hedge advises people not to use armrests while working at a keyboard to maintain the correct angle of arms and wrists. Armrests should be lowered during keyboarding and raised for the sitter to rest the arms periodically and for support while standing up.
For best fit, the armrests in some chairs are adjustable in and out to match body width.
Protecting Disks
The chair should allow the sitter to move to relieve muscles and joints of stiffness. In adults, Vettraino says, the disks between the vertebrae have no blood supply so nutrients flow in and waste is removed only as the disks flex.
Each disk contains a gel which hardens if it remains static for too long. When this happens too much, the disk loses elasticity resulting in the flattening and hardening that constitute deteriorating disk disease, causing pain and muscle spasms.
Movement is facilitated by the chair’s tilting mechanism. When the sitter pushes the backrest back, the seat should tilt up one degree for every two or three degrees of backrest tilt. This opens the angle between the thighs and the spine, which should be greater than 90 degrees, and exerts less pressure under the sitter's thighs than if the seat tilts at the same angle as the back.
The chair should allow the sitter to adjust the pressure needed to tilt the backrest to suit the person’s weight and strength. In most chairs, the tilt can be locked in position, but Hedge says it should be left unlocked and “free-floating” to allow a rocking movement. His research shows the most comfortable and healthiest seated posture is to recline by 100 to 110 degrees.
These adjustments can adapt a chair to the person’s height, weight and leg length, and allow movement that keeps the spine healthy.
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