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Ta k e 5 a n d t r y A B M P F i v e - M i n u t e M u s c l e s a t w w w. a b m p . c o m / f i v e - m i n u t e - m u s c l e s . 101 The motion segment's posterior portion includes the vertebral arches, the facet (zygapophyseal) joints between the adjacent vertebra, transverse and spinous processes of the vertebrae, and the supporting ligaments that span between adjacent vertebrae. The supporting ligaments include the intertransverse, interspinous, and posterior longitudinal ligaments (Image 2). The primary role of the posterior motion segment is to guide and restrain spinal movement in specific directions. The main guide for lumbar spinal movement is the orientation of the facet joints. In the lumbar spine, the facet joints are mostly vertical, allowing the greatest range of motion in spinal flexion and extension. There is some degree of lateral flexion in the lumbar spine and a small amount of rotation. The majority of trunk rotation occurs in the thoracic region. There is minimal flexion and extension in the thoracic region due to the rib cage. The supporting ligaments of the posterior motion segment contribute a great deal to its stability. The intertransverse ligaments span between each of the transverse processes. The interspinous ligaments span between each spinous process. These supporting ligaments act as guy-wires to maintain vertical spinal stability. There is a constant balance between mobility and stability in the spine as we stand on two feet in the vertical gravity plane. Many soft-tissue problems develop from dysfunctional mechanics in the lumbar motion segments. Next, we'll look at some key aspects of lumbar spinal mechanics for the motion segments. BIOMECHANICAL FACTORS All spinal movements involve the coordinated action of adjacent motion segments. When movement is limited at one or more segments, mobility can be lost in those adjacent as well. As a result, coordinated spinal mechanics will suffer. For example, suppose a client has significant hypertonicity in the lumbar spinal extensors between L3 and L5 and corresponding low-back pain. The hypertonicity will limit their ability to properly flex the lumbar spine when bending forward. The limitation in spinal movement during forward flexion causes more anterior pelvic rotation as they attempt to bend forward. The altered mechanics also place increased tensile loads on the hamstrings due to the pelvic rotation, which can make them more susceptible to injury or dysfunction. 1 Anterior components of the motion segment. Image from 3D4Medical's Complete Anatomy application. Anterior longitudinal ligament Vertebral body Intervertebral disk 2 Posterior components of the motion segment. Image from 3D4Medical's Complete Anatomy application. Spinous process Supporting ligaments Vertebral arch Facet joint Transverse process