Loads on Spine - continued
COMPRESSION increases (compared to standing) with sitting, spinal flexion, slouching while sitting.
Slouching involves backward rotation of sacrum compared to ilium, thus widening the posterior L5-S1 intervertebral disc space, and creating strain in ilio-lumbar ligaments.
SHEAR loads are also placed on the SPINE in erect standing. Especially in the lumbar spine, where there is a tendency SHEAR force to cause a vertebra to displace anteriorly over the vertebra inferior to it. The main reason is that, in this region, few spinal extensor muscles lie parallel to the spine, and thus they tend to create both compression and shear loads on vertebral and facet joints.
Fortunately, the SHEAR force of muscle tension acts posteriorly, while the SHEAR force of body weight is acting anteriorly, so the forces usually balance out.