الفهرس 
IntroductionOnly 14 pages are availabe for public view
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Abstract
The human spine is a multi-joint system controlled by muscles which supports the head and trunk and encloses and protects the spinal cord ,each two adjacent vertebral bodies are linked by an intervertebral disc. Together with the corresponding facet joints, they form the ‘Motion segment’. Translation and rotation can occur at each spinal motion segment .Translation occurs when a shear force causes one vertebra to move parallel to the adjacent vertebra. Rotation is the spinning of one vertebra about a stationary axis relative to the adjacent vertebra. Translation and rotation occur at each motion segment during lumbar spine movements in any of the cardinal body planes. Lumbar segmental instability can be defined as loss of spinal stiffness and development of a more elastic one, such that force application to that motion segment produces abnormally great motion compared to that of a normal spine ,so normally tolerated external loads will result in pain, deformity, or place neurological structures at risk . Three subsystems work together and prevent the risk of instability in spine: active subsystem (muscles and tendons), passive subsystem, (intervertebral disc, facet joints, corpus vertebra and ligaments) and neural subsystem.