The influence of slouching and lumbar support on iliolumbar ligaments, intervertebral discs and sacroiliac joints

Chris J. Snijders, Paul F. G. Hermans, Ruud Niesing, Cornelis W. Spoor and Rob Stoeckart

Clinical Biomechanics

May 2004, Vol 19, Issue 4, pages 323-329.

Are you sitting comfortably? Maybe you're sitting too comfortably?

An eminent group of researchers from the Netherlands have investigated the effects of 'slouching' on the mechanics of the lower back and its possible role in low back pain. The study, published in Clinical Biomechanics, develops a long known theory that back sprain and disc herniation can be caused by a prolonged or sudden lumbar flexion, even without elevated loading on the spine, such as during lifting activities. Snijders et al bring this theory to life by quantifying the forces on the lower back during prolonged slouched sitting.

Previous studies had identified an increased incidence of low back pain at home, school and work environments. The authors identified that prolonged sitting was common in all three environments and sought to examine the biomechanical effects of this position. Snijders et al measured relative movements of the L5 vertebra and pelvic bones when moving into a slouch in sitting with chair support at shoulder level. This was achieved using six cadaveric specimens (three male, three female age range 51 – 99 years).

The bodies were gradually moved through lumbar range of movement whilst in a sitting position and these movements were video recorded. From this data the authors calculated biomechanical forces on various anatomical structures in order to estimate a biomechanical model. This is a valid method, but as with all cadaveric research the findings may not be exactly the same as in living biological tissue.

Snijders et al found that sitting slouched against a high back rest forces the spine, 'with a crowbar effect' into a more or less (backwards) convex curve. The high back of the chair acts as the hand on the crowbar, forcing the L5 vertebra to rotate forwards around the Iliolumbar ligament, and levering open the back most side of the intervertebral disc connecting L5 and sacrum. Significiantly, the authors found that a lower backrest eliminates the crowbar effect by lowering the intersection point on the spine of backrest force.

The authors described the characteristic for the crowbar model as forward flexion of the spine combined with backward tilt of the sacrum relative to the pelvis. This loading mode is maximal when back muscle protection against flexion is absent, e.g. in relaxed slouched sitting. In this posture biomechanical model calculations resulted in Iliolumbar Ligament stress near failure load. This has clinical significance because the Iliolumbar Ligament is thought to be a source of mechanical low back pain.

Measurements by Snijders et al demonstrated a significant reduction of movement between L5 and sacrum as well as between sacrum and ilium using a lumbar support. The authors previous work has shown that the use of a chair with a high backrest easily overrules lumbar support due to force exerted on the shoulder blades. Because of this, the authors recommended at least 6 cm free shoulder space from the chair is a minimal requirement. This is because the ribcage is stiff, so back support is not required above the level just below the shoulder blades. However there is still a popular misconception that high backrests provide superior support for seated activities, whether in the office, lecture hall or at the dinner table.

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