During forward bending, a combination of compression, anterior shear, and flexion moment is applied to the lumbar spine due to upper body weight. A combination of posterior muscle and ligament forces must be generated in order to prevent excessive motion and restore upright posture. It is generally believed that forward bending to 90 degrees while maintaining a straight or extended lumbar spine is biomechanically favorable compared to lifting with a rounded back [1]. A simple biomechanical model of the lumbar spine during bending in which the vertical force from upper body weight is balanced with posterior muscle and ligament tension would result in similar levels of compression of the disc regardless of the bending modality. However, this model does not take into consideration the facets. A study involving professional class weightlifters showed that subjects would increase the lordosis in their lumbar spines prior to executing a deadlift maneuver [2]. The authors suggest several possible advantages for why the lifters increased the lordosis including muscle control and geometric advantages, but do not indicate the potential for increased facet engagement. During forward bending the lumbar spine will be exposed to anterior shear forces, which will cause the facets to engage [3]. Posterior muscle activation occurring during facet engagement may generate a fulcrum, which has the potential to reduce the compression experienced by the disc. Therefore, the objective of the current study was to simulate forward bending with a previously validated finite element model of L4-L5 and determine if increasing posterior muscle force results in a reduction in disc pressure. We hypothesized that posterior muscle activation during forward bending would increase facet contact and reduce intradiscal pressure and nucleus extrusion forces thereby minimizing the contribution to progressive disc herniation.

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