Logan Steele
The spine is a flexible column that is supported by a large, complex group of muscles that work together to support the trunk and hold the body upright. The muscles of the spine are also used together in a coordinated manner to create a stable desired position of the spine. The flexor muscles attached to the front of the spine include the abdominal muscles, which allow us to flex, bend forward, lift, and arch the lower back. The multifidus is the largest muscle of the lumbar spine. The psoas minor flexes the trunk at the spinal joints and posteriorly tilts the pelvis at the lumbosacral joint. The major actions involved in spinal flexion are bilateral activity of the rectus abdominis, external and internal oblique, and psoas major.
| Characteristics | Values |
|---|---|
| Muscles that flex the spine | Rectus abdominis, external oblique abdominal muscles, internal oblique abdominal muscles, psoas major, psoas minor, erector spinae, multifidus, semispinalis, rotatores, gluteal muscles, latissimus dorsi, quadratus lumborum |
| Function of flexor muscles | Allow us to flex, bend forward, lift and arch the lower back |
| Muscles that extend the spine | Erector spinae, semispinalis, deep posterior spinal muscles, transversospinalis |
| Function of extensor muscles | Allow us to stand and lift objects |
| Muscles that rotate the spine | Oblique muscles, transversospinalis |
| Muscles that laterally flex the spine | All trunk flexors and extensors (when acting unilaterally), erector spinae, semispinalis thoracis, latissimus dorsi, deep posterior spinal muscles, quadratus lumborum, psoas |
| Other related muscles | Diaphragm, abdominal muscles, hip flexors, spinal extensors, iliopsoas, gluteal muscles, facet joints, multifidus, rotatores, sacrum, PSIS, mammillary processes, vertebral segments, thoracic spine, lumbar spine, interspinal ligaments |
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What You'll Learn
The rectus abdominis, external oblique, and internal oblique abdominal muscles
The abdominal muscles, along with the back muscles, make up the core muscles. These muscles help protect the spine, keep the body stable and balanced, and support the trunk during movement. The rectus abdominis, external oblique, and internal oblique abdominal muscles are three of the five main abdominal muscles.
The rectus abdominis muscle is a paired muscle that runs vertically on either side of the linea alba, a band of connective tissue that divides the two halves of the muscle. It is attached by two tendons: a larger one that connects to the pubic crest and a smaller, medial tendon that attaches to the pubic symphysis. The fibres of the rectus abdominis extend vertically superiorly and insert into the xiphoid process of the sternum and the costal cartilages of the 5th, 6th, and 7th ribs. The rectus abdominis muscle lies within the rectus sheath, which is formed by the merging of the aponeurosis of the transversus abdominis, external oblique, and internal oblique abdominal muscles. This muscle is innervated by the thoracoabdominal nerves, which enter the rectus sheath by piercing its anterior surface. The blood supply to the rectus abdominis muscle is predominantly from the inferior and superior epigastric arteries.
The external oblique abdominal muscles are a pair of muscles, one on each side of the rectus abdominis. They are the largest of the flat muscles and sit at the bottom of the stack. The external oblique muscles run from the sides of the body towards the middle and allow the trunk to twist from side to side. The fibres of the external oblique arise from the fifth through twelfth ribs and run inferomedially. As the fibres approach the midclavicular line, they form an aponeurotic sheath, which travels across the rectus abdominis to the linea alba in the midline.
The internal oblique abdominal muscles are a pair of muscles located on top of the external oblique muscles, just inside the hip bones. They are much thinner and smaller than the external oblique muscles. The internal oblique muscles run from the sides of the trunk towards the middle. Contraction of the internal oblique muscles, along with the external oblique, causes rotation and lateral flexion of the vertebral column.
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The psoas major and psoas minor muscles
The psoas muscle is a long, core muscle that is located deep in the body cavity on either side of the spine. It helps with posture and movement of the hips and upper legs. The psoas major and psoas minor muscles are two distinct muscles that work together to support the body's movement and flexibility.
The psoas major muscle is a long fusiform muscle located in the lateral lumbar region between the vertebral column and the brim of the pelvis. It joins the iliacus muscle to form the iliopsoas, which is a powerful hip flexor. The iliopsoas muscle is surrounded by the iliac fascia and runs across the iliopubic eminence to insert itself on the lesser trochanter of the femur. The psoas major is divided into a superficial part and a deep part, with the lumbar plexus lying between the two layers. The deep part originates from the transverse processes of lumbar vertebrae L1 to L5, while the superficial part originates from the lateral surfaces of the last thoracic vertebra, lumbar vertebrae L1 to L4, and the neighbouring intervertebral discs. The psoas major contributes to flexion in the hip joint and lateral bending of the trunk.
The psoas minor muscle, on the other hand, is a small and inconsistent muscle bundle that is present in around 60-65% of the population. It sits on top of the psoas major and connects to the iliopsoas muscle in the pelvis. The psoas minor flexes the trunk at the spinal joints and posteriorly tilts the pelvis at the lumbosacral joint. In individuals who do not have a psoas minor, the body naturally reabsorbs the muscle tissue and replaces it with another type of supportive tissue.
The psoas muscles are important for maintaining posture and facilitating movement. They can be visualized using magnetic resonance imaging (MRI), electromyography (EMG), or ultrasound. Maintaining the health of the psoas muscles is crucial, as they are susceptible to strains and injuries, which can be prevented through proper stretching and warming up before physical activity.
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Erector spinae, semispinalis, and multifidus muscles
The Erector Spinae, Semispinalis, and Multifidus muscles are all involved in the movement of the spine. The Erector Spinae is a large and superficial muscle that lies just deep to the thoracolumbar fascia. It is composed of three subgroups: iliocostalis, longissimus, and spinalis. The Erector Spinae is responsible for extending, laterally flexing, and ipsilaterally rotating the trunk at the spinal joints.
The Semispinalis muscle is part of the transversospinalis group, which also includes the Multifidus and Rotatores muscles. The Semispinalis muscle assists in the bilateral extension of the trunk, along with the thoracic and lumbar Erector Spinae. The Semispinalis muscle is also involved in trunk lateral flexion.
The Multifidus muscle is the largest muscle of the lumbar spine and is part of the transversospinalis group. It is a group of short, triangular muscles that extend from the cervical to the lumbar spine. The Multifidus muscle aids in the extension, lateral flexion, and rotation of the spine. Bilateral contraction of the Multifidus muscle produces extension of the vertebral column, while unilateral contraction results in ipsilateral lateral flexion and contralateral rotation.
The Erector Spinae, Semispinalis, and Multifidus muscles work together to provide spinal stability and control. In patients with low back pain, there is often decreased activity and atrophy of the Multifidus muscle, compromising spinal stability. The Erector Spinae muscle compensates for this by increasing its activity to stabilize the lumbar spine. However, this increased activity can also increase the risk of injury.
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Transversospinalis, rotatores, and extensor muscles
The Transversospinalis group is a deep group of back muscles that lie deep to the Erector Spinae. It consists of three subgroups: the semispinalis, multifidus, and rotatores muscles. These muscles lie between the spinous and transverse processes of the vertebral column, deep to the erector spinae muscles. They arise from the transverse processes of the vertebral column and run upwards and medially in an oblique fashion to insert into the spinous processes of superior vertebrae.
The semispinalis is the most superficial of the transversospinalis muscle group. It spans the thoracic and cervical regions of the vertebral column, with an attachment to the occipital bone of the skull. The semispinalis cervicis and semispinalis thoracis are innervated by the spinal nerve C3, while the semispinalis capitis is innervated by the greater occipital nerve (posterior ramus of C2 spinal nerve). Bilateral contraction of the semispinalis capitis pulls the head posteriorly, while unilateral contraction pulls the head posteriorly and rotates the chin to the same side of the contracting muscle.
The multifidus belongs to the intermediate layer of the transversospinalis muscle group. This muscle is composed of many short, triangular muscles that span the entire length of the vertebral column, but are thickest and most developed in the lumbar region. The multifidus is divided regionally into three parts: the multifidus cervicis, multifidus thoracis, and multifidus lumborum. The nerve supply to the multifidus muscle is derived from the medial branches of the posterior rami of spinal nerves in the corresponding cervical, thoracic, and lumbar regions.
The rotatores are the deepest of the transversospinalis muscle group. They consist of two parts: the long (longus) and short (brevis) rotatores. The long rotatores pass from the transverse process medially to the spinous process, crossing two vertebrae, while the rotatores brevis pass from the transverse process and insert at the base of the spinous process of the vertebrae above the origin. The rotatores are innervated by the medial branches of the posterior rami of spinal nerves and receive their blood supply via dorsal branches of the posterior intercostal and lumbar arteries. The major function of the rotatores is to stabilize the spine.
The erector spinae is a group of extensor muscles that extend, laterally flex, and ipsilaterally rotate the trunk at the spinal joints. It is composed of three subgroups: iliocostalis, longissimus, and spinalis. The longissimus muscle functions as a powerful extensor of the lumbar, thoracic, and cervical spine, as well as an extensor of the head and neck. The spinalis muscle is the smallest and most medial of the erector spinae muscle group.
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The diaphragm and the abdominal wall
The diaphragm is a sheet of skeletal muscle that separates the thoracic cavity, which contains the heart and lungs, from the abdominal cavity. It is the most important muscle for respiration. When the diaphragm contracts, the volume of the thoracic cavity increases, creating negative pressure that draws air into the lungs. Conversely, when the diaphragm relaxes, air is exhaled by the elastic recoil process of the lungs and thoracic cavity. The diaphragm also helps to expel vomit, feces, and urine from the body by increasing intra-abdominal pressure, and it aids in childbirth.
The diaphragm is innervated primarily by the phrenic nerve, which is composed of fibers from cervical nerves C3, C4, and C5. The diaphragm is supplied by several arteries, including the superior phrenic, musculophrenic, and pericardiacophrenic arteries. Its embryonic development is a complex process involving the coordination of muscle, connective tissue, vessels, and nerves. The septum transversum, composed of mesodermal tissue, forms a separation between the thoracic and abdominal cavities and serves as the foundation for the diaphragm's central tendon.
The abdominal wall, meanwhile, is composed of several muscle groups, including the rectus abdominis, external oblique abdominal muscles, and internal oblique abdominal muscles. These muscles play a crucial role in spinal flexion, particularly during movements such as the curl-up, curl-back, pelvic tilt, and hip lift. Spinal flexion occurs when the spine flexes or the pelvis posteriorly tilts against gravity or external resistance. During spinal flexion, the abdominal muscles work in conjunction with the hip flexors to bring the thigh closer to the pelvis.
The abdominal wall and diaphragm work together to maintain spinal stability and respiratory function. Real-time imaging techniques, such as sonography, can be used to evaluate muscle contraction and relaxation within the abdominal cavity, providing valuable insights into the motion of the diaphragm and abdominal wall.
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Frequently asked questions
The abdominal muscles, including the rectus abdominis, external oblique abdominal muscles, and internal oblique abdominal muscles, are involved in spinal flexion.
The abdominal muscles allow us to flex, bend forward, lift, and arch the lower back. They help bring the trunk forward and can improve spinal alignment issues.
Yes, the hip flexors are also involved in spinal flexion, especially during exercises like the curl-up and curl-back, where they help bring the thigh closer to the pelvis.
Yes, exercises such as the curl-up, curl-back, pelvic tilt, and hip lift specifically target spinal flexion and can help improve spinal alignment.
