The most common cause of low-back pain related to seating is posterior protrusion or extrusion of lower lumbar intervertebral discs.
The normal curve of the lumbar spine in adult man is determined by maintenance of the trunk-thigh and the knee angles at approximately 135 degrees.
Alteration of this normal lumbar curve, either an increase in standing erect or a decrease in sitting or stooping, is caused largely by the limited length and consequent pull of the trunk-thigh muscles of the opposite side.
The most important postural factor in the causation of low-back pain in sitting is decrease of the trunk-thigh angle and consequent flattening of the lumbar curve.
The next most important cause of low-back pain in sitting is lack of primary back support over the vulnerable lower lumbar intervertebral discs.
Added factors of comfort in seating are the shortness of the seat, a rounded narrow front border, an open space beneath for better positioning of the legs, and permissive change of position in the seat.
The design of all seats, regardless of model or size, should be based on this knowledge.
Division of Neurological Surgery, Department of Surgery, University of Nebraska College of Medicine, Omaha