• Chris J Snijders, Paul F G Hermans, Ruud Niesing, Cornelis W Spoor, and Rob Stoeckart.
• Department of Biomedical Physics and Technology, Faculty of Medicine, Erasmus MC, University Medical Center Rotterdam, P.O. Box 1738, 3000 DR Rotterdam, The Netherlands. c.snijders@erasmusmc.nl
• Clin Biomech (Bristol, Avon). 2004 May 1; 19 (4): 323-9.

ObjectiveTo investigate lumbopelvic kinematics when moving into a slouch.DesignA biomechanical model was developed. Load tests in vitro verified the model.BackgroundThe precise mechanism causing disc herniation and back sprain is still debated. Most biomechanical studies have focused on lifting in a stooped posture. Previous studies address instability situations due to Euler buckling of the spine under axial load. However, no studies address lumbosacral, iliolumbar and sacroiliac kinematics in slouching, i.e. flexing the spine in situations with negligible compressive spinal load.MethodsModeling started with the click-clack movement, i.e. the transition from lumbar lordosis to lumbar kyphosis by the combination of backward rotation of the pelvis and ventral flexion of the spine. The flexed spine was compared with a crowbar which uses the iliolumbar ligaments as fulcrum and pivot. To analyse the click-clack movement in sitting, unembalmed erect human trunks were moved from a forward position to a backward position, recording angular changes between L5, sacrum and ilium.ResultsWhen moving the trunk stepwise backward with support at shoulder level, L5 showed forward rotation with respect to the sacrum, but rotation of the sacrum with respect to the iliac bones was reversed (i.e. counternutation). L5 showed displacement in ventral direction with respect to the ilium. Measurements were in agreement with prediction from the crowbar model of the spine.ConclusionsBackward rotation of the pelvis combined with flexion of the spine, i.e. slouching, results in backward rotation of the sacrum with respect to the ilium, dorsal widening of the intervertebral disc L5-S1 and strain on the iliolumbar ligaments when protection from back muscles against lumbar flexion is absent. Lumbar backrest support almost eliminates lumbosacral and sacroiliac movement.RelevanceUnderstanding why the iliolumbar ligaments are loaded in slouching contributes to the understanding of the biomechanics of low back pain in everyday situations with small or negligible compressive spinal load. The results recommend lumbar support: backrests with free shoulder space.

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