You Asked for It
April 2003

(Because of copyright laws, I cannot reproduce the original question, but I can summarize it and give you my original response).

Question:
A worker complained about feeling like she was slipping forward in the chair after a lumbar support had been added. The seat was at a slight positive tilt and the lumbar support had been added in response to concerns about back pain.

Answer:
Adding a lumbar support to the chair may have decreased the available depth of the seat pan. When this happens, the worker feels like she is slipping out of the chair. There should be about a 3 finger gap between the back of the knee and the front edge of the seat pan. This will protect the delicate area at the back of the knee while providing good support through the thighs. The thickness of the lumbar pad should be geared to the needs of the worker to fill in the area between the buttocks and the outward curve of the thoracic spine. As a simple guide, the lumbar support should be no thicker than the forearm muscles when gently squeezed between the thumb and forefinger. A heavier person will have a thicker forearm and likely need more lumbar support and a thinner person the opposite. The exception would be with individuals involved in weight training, of course.

The other factor is the positive slope to the seat pan. Because the thigh is wedge shaped, I find that keeping the seat pan horizontal is sufficient to create a slight slope to the surface of the thighs. This will place the mid point of the knee slightly below the middle of the hip. The flat seat pan will also give a greater sense of physical security in the chair. This is particularly important when the worker is wearing slippery garments, such as pantyhose, slips or lined pants.

Adding a foot rest may only exacerbate the problem because of the flexibility of the supporting surface. When a worker feels unstable on the chair and at their feet, the tendency is to flex the back slightly, hunching forward. This may be part of a fight or flight response. Increasing stability will allow the worker to relax, encouraging use of the backrest.

[As an additional note, another responder to the inquiry made a very good point about not dithering around with lumbar supports and additions to the chair - just get a chair that fits the person.]

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