Lateral flexion is the anatomical term for the movement of bending sideways at the waist. Alternately known as side bending or lateral trunk flexion, it can refer to movement of the entire spine in a lateral direction, of the neck only, or of a segment of the spine. Lateral flexion is made possible by cartilaginous joints between adjacent vertebrae in the spinal column, which unlike most of the body’s movable joints function not as individual units of movement but as part of movement segments producing motion in entire sections of the spine. It is similarly facilitated not by individual muscles but by groups of closely situated muscles firing simultaneously.
When a person bends sideways from the waist, motion is occurring in multiple joints at once, just as it does when one tilts the head to the side. This is because lateral flexion affects entire portions of the spine, not just two adjacent vertebrae. Side bending from the waist involves movement in both the thoracic and lumbar spine; laterally flexing the head requires the involvement of most of the cervical spine. The smallest movable unit that is two adjacent stacked vertebrae and the cartilaginous disk between them is known as a spinal motion segment or functional spinal unit. Lateral flexion depends upon the coordination of several motion segments.
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Each individual motion segment is capable of a varying degree of lateral flexibility. With multiple segments bending sideways at the same time, a larger degree of lateral flexion is visible than that seen at individual segments, with the cervical spine possessing the most lateral flexibility and the lumbar spine the least. Between L1 and L2, for instance, the first two lumbar vertebrae, is approximately six degrees of lateral flexibility; the same is observed between L2 and L3. L3 and L4 have a greater range of motion between them — eight degrees — while six degrees are possible between L4 and L5, the bottom lumbar vertebra.
Lateral flexion would not be possible if not for the action of many muscles of the trunk and neck. Muscles that produce this motion tend to originate on the side of one vertebra or vertebrae and attach to the side of another vertebra or vertebrae elsewhere in the spine, so that by contracting these muscles shorten the spine along one side and pull the trunk laterally. The muscles of the erector spinae group deep in the back comprise multiple vertical segments that tend to correspond with the cervical, thoracic, or lumbar region.
The longissimus cervicis has fibers originating on the transverse or side processes of T1 through T5 in the thoracic spine. These fibers ascend and attach individually to the same processes on C2 through C6 in the cervical spine. These muscles also only produce lateral flexion when firing unilaterally, or on one side only. When firing bilaterally they instead extend the spine and help to maintain upright posture.