This document discusses the biomechanics of the spine. It describes how the nucleus pulposus imbibes water, developing internal pressure that is exerted in all directions against the annulus and end plates. This increases the stiffness of the end plate and annulus fibrosus. Measurements find intradiscal pressure is highest with activities like lifting and decreases with loads closer to the body. The spine undergoes shear and tensile stresses during bending, axial rotation, and extension. Low back pain is a frequent cause of activity limitation and has direct annual costs of $11.4 billion, often associated with mechanical loading conditions like frequent bending and twisting.