Musculature of the Lumbar Spine

The lumbar musculature is important to spinal stability, balance, weight distribution, static postures, and assists and limits flexion, extension, rotation, side bending, and combinations of movement. To more fully understand the musculature of the lumbar spine, a brief overview of directional terms and anatomy is needed. Several terms used in the sections about spinal muscles will be more familiar if this preliminary anatomical review is considered.

Common directional terms include: Anterior - toward the front of the body, posterior or dorsal - toward the back of the body, medial -toward the body's midline, lateral or side - way from the body's midline, superior - upper or above, and inferior - lower or below. Anatomical planes are invisible flat surfaces. These terms refer to body regions or sections, such as: The coronal or frontal plane vertically divides the front and back of the body. The sagittal or lateral plane vertically divides the left and right sides of the body. The axial or transverse plane horizontally divides the upper and lower body.

The figure illustrates the body's anatomical planes. Notice the coronal, sagittal and axial planes.

The structure of the lumbar spine supports and connects the lower and upper body, assists balance and weight distribution, and enables and limits movements such as flexion, extension, side bending, rotation, or a combination. Although the lumbar muscles are important to spinal stability during rest and activity, the musculature is only one part of other supportive and complex structures. Bones, joints and discs serve as bases of attachment for spinal musculature, nerves innervate muscles to action, arteries and veins deliver nourishment, and ligaments provide support and protect excessive movement.

The slides that follow provide information about the lumbar spine's structures and include the vertebrae; vertebral arches; facet or zygapophysial joints; intervertebral discs; neural structures including the spinal cord, nerve roots, and cauda equina; vascular system; ligaments, and muscles.

There are 5, and sometimes 6, lumbar vertebrae in the low back. The lumbar vertebrae are the largest and most dense bones in the spinal column. The lumbar vertebrae carry and distribute most of the body's weight during rest and activity. Illustrated is an axial and lateral view of a lumbar vertebra.

The levels of the lumbar vertebrae are abbreviated L1 through L5, or L6. The sacrum and coccyx follow the last lumbar level. However, the bones in the sacrum and coccyx are not vertebrae. The sacral and coccyxgeal bones may be joined or naturally fused together in adults. The sacral bones are abbreviated S1 through S5 and the coccyxgeal bones, or tailbone, are not abbreviated. The figure illustrates the regions of the lumbar spine.

Together, the anterior and posterior arches create the spinal canal, or foramen – a hollow, or hole that contains and protects the spinal cord or cauda equina. The arches also provide a base of attachment for the spine's ligaments and musculature. The anterior arch is the front part of the vertebra. The posterior arch is the rear part of the vertebra and includes the pedicles, laminae, and processes.

Facet joints, or zygapophysial joints, are synovial joints, meaning each joint capsule is nourished and lubricated by synovial fluid. Joint capsules are surrounded by connective tissue. Each articulating joint is covered by cartilage. The illustration shows the facet joints in relation to the intervertebral disc and vertebral bodies.

Each vertebra has 2 sets of facet joints; one on the left and right sides of each vertebral body. The superior articular and inferior articular facets work like hinges and posteriorly connect vertebral bodies. Illustrated, are facet joints in motion during flexion and extension.

Between each lumbar vertebral body is an intervertebral disc. Discs function to add height to the vertebral column, absorb and distribute shock during rest and activity, allow spinal movement and range of motion, and serve as base of attachment for spinal ligaments.

The spinal cord, cauda equina and nerve roots are the lumbar spine's primary neural elements. The spinal cord ends near L1-L2 and becomes the cauda equina. The cauda equina is a bundle of intradural nerve roots and rootlets resembling a horse's tail. The figure illustrates the cauda equina and demonstrates spinal nerves exiting the lumbar and sacral spine.

The spinal nerve roots branch off the spinal cord in pairs. The nerve roots pass through the neuroforamen, or nerve passageways naturally created on either side of the vertebrae. The spinal nerves branch outward beyond the spinal canal and create the peripheral nervous system. In each of the lumbar and sacral levels, there are 5 pair of spinal nerves and one pair in the coccyx. The spinal nerves, including the cauda equina, innervate the lower body and extremities, including muscles. Nerves enable movement, or motor functions, and feeling, or sensory functions.

A complex system of lumbar arteries and veins provide nourishment to and remove waste from spinal structures such as bones, nerves, and muscles. The lumbar arteries include the lumbar segmental. The left and right common iliac arteries feed the lumbar region, pelvic organs, and legs. The segmental artery runs from the lumbar spine to the sacrum. Lumbosacral arteries include the middle sacral, iliolumbar, and internal iliac. The lumbar veins include the inferior vena cava in the lumbosacral region that returns blood to the heart. The azygous, in the lumbar region, returns blood when the inferior vena cava is obstructed. The hemiazygous, lumbar segmental, left and right common iliac are in the lumbar region. Batson's plexus, also in the lumbar region, provides an alternate route to the heart. The common iliac is found in the lumbosacral region.

The vasculature of the lumbar region is illustrated. The figure on the left is an anterior view of the vasculature structures. The arteries and veins are pulled away in the figure on the right to reveal the several lumbar vertebrae and discs.

Ligaments are fibrous bands of connective tissue, such as fascia, that link 2 or more bones, cartilages, or structures together. Their purpose includes joint stability during rest and movement, limiting excessive movements such as hyperextension or hyperflexion, and prevention of certain directional movements. The figure depicts low back pain commonly caused by hyperextension or hyperflexion.

Basically, there are 4 types of lumbar ligaments: Ligaments that interconnect the vertebral bodies, ligaments that interconnect the posterior elements, iliolumbar ligament, and false ligaments. The posterior elements include the posterior arches, facet joints, laminae, and spinous processes.

The figure illustrates several of the spine's ligaments. Included are the ligamentum flavum, anterior longitudinal, posterior longitudinal, interspinous, and supraspinous ligaments.

Where each ligament attaches to the spine and its functions is presented. The ligamentum flavum is essentially an elastic ligament composed of 80% elastin and 20% collagen. It connects beneath the facet joints and joins each vertebral laminae and limits flexion. The anterior longitudinal ligament attaches anteriorly to the vertebrae and discs and extends onto the sacrum. It limits extension and reinforces the front of each annulus fibrosis. The posterior longitudinal ligament attaches posteriorly to the vertebrae and discs. It limits flexion and reinforces the back of each annulus fibrosis. The interspinous ligaments connect adjacent spinous processes and limits flexion. The supraspinous ligament attaches to the rear edges of spinous processes and limits flexion. The anterior, superior, posterior, inferior and vertical iliolumbar ligaments connect the transverse process of L5 to the ilium, which is part of the pelvis. It aids spinal stability and limits some movement. The false lumbar ligaments are the intertransverse, transforaminal, and mamillo-accessory ligaments. Rather than true ligaments, they may form parts of complex fascia systems.

Skeletal muscle is striated, meaning its cells, or fibers, contain bands called striations of dark and light tissue. The light-colored tissue is fat. Higher concentrations of myoglobin are responsible for the darker color. Myoglobin is an oxygen storehouse for mitochondria; each cell's power plant. Myofibrils are muscle proteins that enable muscles cells to contract; contraction is the primary action of a muscle. Muscle fibers are not all alike as some contract at different speeds. The contraction speed is partially dependent on the muscle fibers' ability to split Adenosine Triphosphate, or ATP. Structural attributes, including fatigue onset, and cell metabolism, are variables that affect muscle function.

The muscles of the lumbar spine can be divided into 4 functional groups. Extensors enable straightening, forward flexors enable forward bending, lateral flexors enable side to side bending, and rotators enable twisting or rotational movements. Some of these muscles attached directly to the spine while others work to exert an action on the lumbar spine. For example, the longissimus and iliocostalis lumborum do not attach to the lumbar spine, but attach to the ilium (pelvis) and sacrum from the thoracic level.

The illustration does not show the entire musculature of the lumbar spine. However, it provides a good overview of the spine's deeply layered musculature support system. Of note are the rotares, quadratus lumborum, intertransversarii, interspinalis, and iliocostalis lumborum.

The extensor muscle group enables the spine to straighten upright, such as from a bent forward position. There are several layers of extensor muscles in the low back. The interspinales are 4 paired muscles on the left and right sides of the interspinous ligament, which connect to the spinous processes. With assistance from the multifidus, the interspinales aids posterior sagittal rotation of the superior vertebra. The intertransversarii mediales are somewhat similar in size to the interspinales. These muscles attach to the rear of the superior articular process of each lumbar vertebrae and aids in rotation and extension. The multifidus originates from the sacrum and attaches to posterior processes and acts as a spinal column extensor and rotator. The lumbar erector spinae, or sacrospinalis, lies lateral to the multifidus and creates the dorsolateral aspect of the low back's musculature. The erector spinae is made up of the longissimus thoracis and iliocostalis lumborum. The lumbar intermuscular aponeurosis, a continuation of the erector spinae, separates the longissimus and iliiocostalis, which assists in spinal column extension and rotation.

The forward flexor muscle group consists of 2 muscle groups: the Iliothoracic, such as the abdominal muscles and femorospinal, the psoas major and iliacus muscles. The psoas major originates at the L1-L5 transverse processes, T12-L5 bodies, and intervertebral discs below T12-L4. Although its attachment is spinal, the muscle action flexes and laterally rotates the hip. The iliacus originates within the iliac, or pelvis within the abdomen, and serves to flex the hip.

Lateral flexion of the trunk combines side bending and rotation. The quadratus lumborum, as seen in the illustration, is paramount to this movement. The muscle originates near the inferior border of the 12th rib, follows the transverse processes of L1-L4, connecting to the iliolumbar ligament and iliac crest, the pelvic area.

The extensors and lateral flexors assist in rotation. The extensors include the interspinales, intertransversarii mediales, multifidus, and lumbar erector spinae or sacrospinalis. The quadratus lumborum is a lateral flexor.