Sitting and Low Back Pain
*(Note: this article is probably too technical for the average reader.)
People who sit a lot have more lower back problems(1) and more disc degeneration.(2) This results from the fact that prolonged sitting may result in excessive loading and tissue deformation of the lumbar spine. Some ways of sitting mat be more harmful on the back than others. What is the best way to sit to reduce excessive stresses on the spine?
There has been a controversy in the research literature over whether it is better to sit with a flattened or an arched back. Some have argued that sitting with a flattened back results in increased nutrition into the discs and less load on the posterior discs, where we often have problems. Adams and Hutton(3) argue that flexed postures are advantageous, since flexion results in increased fluid flow and improved transport of nutrients into the intervertebral discs. This may decrease the likelihood of degenerative changes, which have been linked with decreased metabolic transport in the disc.(4)
In addition, a flexed posture is beneficial because it lowers the stress on the apophyseal (facet) joints and on the posterior portion of the annulus fibrosus, and decreases the likelihood of injury to the spine when subjected to compressive forces. James Cox echoes this view and concludes that "it is mechanically and nutritionally advantageous to flatten the lumbar spine when sitting and when lifting heavy."(5) Adams and Hutton and Cox both point to the population study by Fahrni that looked at primitive cultures who squat while sitting, a posture that flattens the lumbar spine, and found decreased incidence of degenerative changes in their lumbar spines.(6)
On the other hand, spine experts such as Stuart McGill and Robin McKenzie have argued that maintaining lordosis most of the time while sitting is preferable since this avoids stressing the posterior ligaments and helps to prevent posterior disc creep. As the spine is flexed, the ligaments become loaded and the back muscles shut off.
McGill has shown that sitting with the back slouched for as little as 20 minutes can result in increased laxity of the posterior spinal ligaments. It may take 30 minutes or more for these ligaments to regain their previous level of stiffness.(7) McKenzie had discussed this overstretching of the ligamentous structures with sitting in a flexed posture, though it was not until McGill's study that his theory was scientifically demonstrated.(8)
Research has also shown that lumbar disc herniation may result from prolonged sitting in the typical flexed posture.(9) This is especially the case if sitting occurs in motor vehicles where the vibrational forces add to the stress on the discs.
McKenzie also argued that sitting with the spine in extension reduces the intradiscal pressure. Extension helps to cause the nucleus to move anteriorly, thus helping to prevent and to reduce the tendency for posterior disc bulging and protruding.(8, p.7)
A recent study supports the lordotic position for sitting as preferable since such a posture helps to balance the loads on various spinal structures.(10) This was an in vitro study in which 12 lumbar spines were subjected to constant loading conditions while in flexed and extended seated postures. The specimens were loaded for 30 minutes with 500 newtons and forces on the anterior column and the facets were measured. Forces on the posterior ligaments, the disc shear and the facet impingement forces were computed via a quasi-static analysis from the data.
The results of this study were as follows:
The authors of this study (Hedman and Fernie) concluded that the "minimization of disc shear, tolerable levels of ligamentous tension, lower disc loads and a balancing of facet impingement and articular fact forces were found to be characteristics of prolonged erect sitting in this study. Based on these results, one would expect that the extended seated posture would reduce exasperation of tissues as compared to flexed postures." They also concluded that the increased progression of forces in the tissues of the lumbar intervertebral joints in the flexed seated posture would likely result in increased degenerative changes in these joints.
Hedman and Fernie also note that the reduced disc narrowing found in the population studies of Farnhi may be attributable to other factors such as diet, environmental purity, absence of cigarette smoking, absence of vehicular vibration, amount of exercise, and types of routine physical activity.
The conclusion to be drawn from this study is that the lordotic, extended position should be preferred for sitting over the flattened or flexed posture. However, given that any sitting posture results in increased spinal loading, frequent changes in posture from sitting to standing, etc., over the course of a day is to be preferred for protecting the spine.