forces acting on the spine include body weight, tension in the spinal ligaments and paraspinal muscles, intra-abdominal pressure, and any applied external loads. When the body is in an upright position, the major form of loading on the spine is axial, and the lumbar spine supports the weight of the body segments above it. Although most of the axial compression load on the spine is borne by the vertebral bodies and disks, the facet joints, when the spine is in hyperextension, may bear as much as approximately 30% of the load. Under significant compressive loading, such as during a heavy lifting task, increases in intra-abdominal pressure occur that may help to stiffen the trunk to prevent the spine from buckling (8).