If you watch your back in a mirror while you stand and twist, you will see a surprising amount of twisting taking place in the thoracic region: 30-40" of rotation in each direction between Ti and Li in an average young adult. The design of the anterior and posterior functional units of the vertebral column (chapter 4) permits these movements. To summarize: the anterior functional unit of the spine is a flexible rod composed of the stack of vertebral bodies and intervertebral disks, and the posterior functional unit is a tube 'enclosing the vertebral canal) that is composed of the stack of verterbral arches, superior and inferior articulating processes, and various restraining 'igaments. In general, the anterior functional unit permits twisting and ^ending, and the posterior functional unit restricts twisting and bending.
The thoracic region of the spine permits a lot of twisting for three reason First, it contains twelve vertebrae and intervertebral disks—half tl vertebral column—and this means that there are twelve sites for rotatii 1 (fig. 4.7). Second, the axis of rotation for twisting in the chest runs appro mately down the center of the anterior functional unit (fig. 4.6a, large d< i, permitting the hydraulic system of each intervertebral disk (fig. 4.11) to w( k perfectly, compressing its nucleus pulposus axially, stretching the ela; ¡c fibers of its annulus fibrosus evenly all around its perimeter, and causing ie disk to bulge moderately on all sides. The elastic fibers in the annulus fibre is that are oriented obliquely will be either stretched or released depending >n their orientation and the direction of the twist, and those that are oriei ¡d vertically and horizontally will be stretched evenly all around.
A third reason twisting takes place easdy in the thoracic region is at the synovial joints for the superior and inferior articulating processi of each thoracic vertebra are oriented in a frontal plane, one roughly par lei to the back surface of the chest, and this orientation allows the joints to lip efficiently with respect to their neighbors above and below (figs. 4.6-7
The large number of vertebrae and intervertebral disks, the ideal of rotation, and the ideal orientation of the articular processes—all of t se facilitate thoracic twisting. And the numbers add up. With twelve pos ble sites for rotation, we need only an average of 30 between adjacent verte 1 ae to make a total of 36°, and this means that twisting here requires nly slight adjustments between adjacent vertebrae and their joints. In fan the articular processes in this region can be displaced from one anoth so readily that it is not the spine but the rib cage that is the main lii to twisting. Were it not for restrictions there, this part of the spine uld probably rotate 120° in each direction.
Like the articular processes, the ribs are also roughly oriented n a transverse plane, and only a slight shearing effect between adjacent 1 >s is needed to allow a small amount of spinal rotation between adjacent vei bral bodies. And this is what accounts for the 30-40" of rotation that we at ally experience in the chest. Beyond 40°, the rib cage becomes the main imped ient to twisting because the ribs connect to the sternum in front by way the costal cartilages, creating a stabilized cage that can rotate only so far.
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