 While sitting or bending to lift, internal pressure on a disc can move from 17 psi (lying down) to over 300 psi (lifting with a rounded back).Herniation of the contents of the disc into the spinal canal often occurs when the anterior side (stomach side) of the disc is compressed while sitting or bending forward, and the contents ("nucleus pulposus") get pressed against the tightly stretched and thinned membrane ("annulus fibrosus") on the posterior side (back side) of the disc.The combination of membrane-thinning from stretching and increased internal pressure (200 to 300 psi) results in the rupture of the confining membrane.