In order for the spine to control movement and assume responsibility for balance under the many stresses of physical activity, spinal muscles attach themselves to many different vertebrae and their parts, as well as to the arms and legs, the head, the rib cage, and the pelvis. Movements of the spine include flexion and extension (forward and back bending), lateral (left and right side) bending, and rotation (twisting). The muscles of the spine act behind the spine to extend it, in front of the spine to flex it, or in combination or different amounts on each side to create side bending and twisting.
Extension of the spine is defined as movement that produces backward bending. The spinal extensor muscles lie behind the vertebral column and span its entire length. There are a number of named muscle groups which extend the spine. Each layer of muscles has specific subdivisions in the neck, midback, and lower back regions.
The deepest extensor muscles have the shortest fibers which often span only one or two vertebrae, though the muscle itself may be extensive, as with the multifidus; those extensor muscles located more superficially may span many vertebrae and have multiple attachments. Because many spinal muscles overlap in different planes, the spine is well equipped to provide stability during all types and degrees of movement, but there is also ample opportunity for painful events or spasm when any muscle fibers become fatigued or over stretched.
The erector spinae is the most important extensor muscle group of the back and is responsible for returning the vertebral column to its "erect" position following motion. Its tendinous origin is extensive, arising from the top of the pelvis (iliac crest), most of the lumbar vertebrae, and several of the lower thoracic vertebrae. The subdivisions of the erector spinae blend in the lower back, and in so doing, offer great strength to the lumbar area. The appearance is that of one common back muscle. However, the subdivisions of the erector spinae group in the upper back become more muscular and the fibers noticeably separate into three smaller groups as the iliocostalis, longissimus and spinalis muscles. The erector spinae muscles secondarily contribute to lateral bending, rotation of the spine, and movement of the head. Another name for this muscle group is the sacrospinal muscle group. [Figure 1 ]
Figure 1: Spine muscles for extension of spine, back view
Flexion of the spine is defined as movement that produces forward bending, such as bringing the chin toward the chest or bending forward at the waist as if to pick up an object. Flexor muscles are located in the cervical and lumbar regions of the spine; there are no flexor muscles in the thoracic region. The psoas muscle (from the inner thigh bone to the lumbar vertebrae) flexes the trunk at the hip and has little effect on flexing the spine itself. The main lumbar spine flexors are the front muscles of the abdominal wall, the rectus abdominis, and the abdominal internal and external oblique muscles. The abdominal muscles are not directly attached to the spine, but importantly contribute to spinal movement by compressing the stomach organs and flexing the spine. Other trunk muscles (quadratus lumborum, trapezius, latissimus dorsi) also participate in flexion of the vertebral column, shoulder or head movements, or arm movement. Trunk muscles, unlike the abdominals, attach to the spine. [Figure 2 ]
Figure 2: Abdominal muscles (left) and trunk muscles (right) for flexion of spine
When the extensors and flexors contract on only one side, lateral bending of the spine occurs. The quadratus lumborum is a muscle that contributes mainly to lateral bending of the lumbar spine and is well situated for that purpose, spanning from the pelvis and lower lumbar area to the ribs and upper lumbar vertebrae.
Rotation occurs only in the neck region of the spine and in the thoracic region (middle back, or rib cage area). Little or no rotation occurs in the lower back, as this motion is blocked by the position and orientation of the facet joints. While there are small muscles which act directly on the spine for rotation, the most powerful rotator muscles are the abdominal internal and external oblique muscles whose fibers run obliquely to the long axis of the body. The multifidus is a long muscle from the transversospinalis group; most of its fibers extend from one of the vertebral processes to the spinous process of a vertebra several levels above. It contributes to extension and lateral rotation of the spine