Functional Anatomy of the Lumbar Spine and Abdominal Wall

Original Editor - Ewa Jaraczewska
Top Contributors - Ewa Jaraczewska, Jess Bell, Robin Tacchetti, Kim Jackson and Lucinda hampton

Introduction

The lumbar spine supports much of the upper body's weight. Intervertebral discs maintain the spaces between the vertebrae. These discs act like shock absorbers throughout the spinal column to cushion the bones as the body moves. Ligaments hold the vertebrae in place, and tendons attach the muscles to the spinal column.

The skeletal support of the abdominal wall includes the vertebral column and lower ribs. The abdominal wall contains skin, fascia, and muscle. It provides coverage for the organs, generates intra-abdominal pressure, and moves the vertebral column. This article discusses the key anatomical structures of the lumbar vertebrae and abdominal wall, including the bony structures, articulations, ligaments, muscles, nerves and the vascular supply.

Key Terms

Axes: lines around which an object rotates. The rotation axis is a line that passes through the centre of mass. There are three axes of rotation: sagittal passing from posterior to anterior, frontal passing from left to right, and vertical passing from inferior to superior. The rotation axes of the foot joints are perpendicular to the cardinal planes. Therefore, motion at these joints results in rotations within three planes. Example: supination involves inversion, internal rotation, and plantarflexion.

Bursae: reduce friction between the moving parts of the body joints. A bursa is a fluid-filled sac. There are four types of bursae: adventitious, subcutaneous, synovial, and sub-muscular.

Capsule: one of the characteristics of the synovial joints. It is a fibrous connective tissue which forms a band that seals the joint space, provides passive and active stability and may even form articular surfaces for the joint. The capsular pattern is "the proportional motion restriction in range of motion during passive exercises due to tightness of the joint capsule."

Closed pack position: the position with the most congruency of the joint surfaces. In this position, joint stability increases. For example, the closed pack position for the interphalangeal joints is full extension.

Degrees of freedom: the direction of joint movement or rotation; there is a maximum of six degrees of freedom, including three translations and three rotations.

Ligament: fibrous connective tissue that holds the bones together.

Open (loose) pack position: position with the least joint congruency where joint stability is reduced.

Planes of movement: describe how the body moves. Up and down movements (flexion/extension) occur in the sagittal plane. Sideway movements (abduction/adduction) occur in the frontal plane. The transverse plane movements are rotational (internal and external rotation).

Lumbar Spine Structure

Lumbar vertebrae

There are usually five lumbar vertebrae, which are known as L1-L5. However, 10–17.4% of adults present with a 6th lumbar vertebra (or a sacral vertebra). Having an additional vertebra may influence sagittal parameters. For instance, it may result in a diminished sacral tilt (i.e. the sacrum is in a more vertical position). Individuals who have additional lumbar vertebrae may also have fewer thoracic vertebrae (e.g. 11 rather than 12 thoracic vertebrae).

The lumbar vertebrae are located between the 12th (or last) thoracic vertebra and the sacrum bone. They are the largest vertebrae of the human body. Each lumbar vertebra contains the following structural elements:

• Vertebral body
  • Kidney-shaped
  • Wider side to side than front to back
  • The posterior aspect of the vertebral body changes from concave to convex
• Vertebral foramen
  • Triangular in shape
  • Larger than in the thoracic vertebrae, but smaller than in the cervical vertebrae
• Two pedicles
  • Posterior element of the lumbar vertebra
  • Attach to the cranial half of the vertebral body
  • Change in shape from L1-L5
• Two laminae
  • Flat and broad
  • Two pedicles and two laminae form the vertebral arch
  • Centrally connect to the spinous process
• Spinous process
  • Projects perpendicularly from the body
  • Short and sturdy
  • Described as "hatchet-shaped"
  • Provides an attachment point for muscles and ligaments
• Two transverse processes
  • Located on the posterior surface
  • Project laterally on each side of the vertebra
  • Long and slender
  • Provide attachment points for muscles and ligaments
  • L1-L3 project horizontally
  • L4-L5 project upward
• Four articular processes
  • Two superior articular facets and two inferior articular facets
  • The point where two facets join is called the zygapophyseal joints

Lumbar Lordosis

Lordosis = the natural curve for the lumbar spine. This curve defines lumbar spine alignment in the sagittal plane. However, the pelvis is "the cornerstone of spinal sagittal alignment". The curve of the sacrum determines the lumbar curve.

Optimal sagittal alignment of the lumbar spine offers the following benefits during locomotion:

• Allows the body's centre of mass to be positioned above the hip, knee, and ankle joints
• Minimises the muscular force used for postural control

Assessment of lumbar spine alignment must include an analysis of the alignment of the lower limbs. For instance, measuring the degree of knee flexion and pelvic shift might help clinicians identify compensatory mechanisms resulting from sagittal plane malalignment.

Abdominal Wall Structure

The skeletal support of the abdominal wall comes from the vertebral column and lower ribs posteriorly. Anteriorly, it connects to the thoracic cage (rib cage) superiorly and the pelvic bones inferiorly. It is defined by the following landmarks:

• superior border = the xiphoid process and costal cartilage of ribs 7–10
• inferior border = the umbilicus in the middle, and inguinal ligament, pubic crest and pubic symphysis

You can read more about pelvis anatomy here.

Bones, Articulations and Kinematics of the Lumbar Spine

Bones and Articulations

The lumbar spine has two types of joints located between each lumbar vertebra:

1. intervertebral disc joint
2. zygapophyseal joint or facet joint

Bones	Articulations	Characteristics	Key palpation points
Vertebral body	Intervertebral disc joint	This joint is formed between the inferior surface of one vertebral body and the superior surface of the vertebral body that lies below (for example, the inferior surface of L1 and the superior surface of L2). An intervertebral disc separates the vertebral bodies.	To find each segment of the lumbar spine, position the patient prone. The location of the spinous processes will determine the location of each segment. You can palpate the spinous processes in the centre of the patient's back. Start with finding T12 by palpating the patient's lowest rib and following your finger along the border of the rib towards the centre of the spine. The T12 spinous process is in line with the 12th rib (which inserts into T12 spinal segment). Below T12, you can palpate L1 and continue along the spinous process of L2, L3, and L4. If you start your palpation from the distal end of the lumbar spine, you must consider the following: the spinous process of L4 "is usually determined by the position of the highest point on both sides of PSIS", but the accuracy of palpation is only 36%.
Articular processes	Zygapophyseal joint (Facet joint)	Formed between the articular processes of each lumbar vertebra. Located on the left and right sides of the spine. They link one vertical segment to the other. For example, the facet joint between L3 and L4 will connect L3 to L4. The superior articular surface is vertical and concave, and the inferior is vertically convex. Their primary role includes the stabilisation of the spinal motion segment. They also contribute to axial compressive load transmission.	The facet joint CANNOT be directly palpated due to overlying soft tissue. When you move your fingers two to three centimetres in the lateral direction from the spinous process, you will be on the muscles overlying the facet joint.

Kinematics

The lumbar spine is considered a three-joint complex or an articular triad. The zygapophyseal (facet) joints and intervertebral disc joint are part of a "spinal motion segment". The various parts of the spinal motion segment work together to allow spinal motion and protect the spine from excessive movements.

The spinal motion segment has the following characteristics:

• The intervertebral disc joint transmits mostly vertical compressive loads
• The facet joints guide and stabilise the motion segment
• In an erect standing position and erect sitting posture, spine load is also transmitted through the facet joints

Joint	Type of joint	Plane of movement	Motion	Kinematics	Closed pack position	Open pack position
Intervertebral disc joint	Cartilaginous joint, symphysis	A single joint: structurally fused but allows very limited, multidirectional movements All lumbar intervertebral disc joints and zygapophyseal joints combined together: Saggital Frontal Transverse	Single joint: Translating, tilting, rocking and compressing movements. The following movements occur when all lumbar intervertebral disc joints and zygapophyseal joints are moving together : Flexion/extension Lateral flexion Rotation	The following movements occur when all lumbar intervertebral disc joints and zygapophyseal joints moving together : Flexion 60 degrees Extension 25 degrees Lateral flexion 20-30 degrees Rotation 1-5 degrees	Full extension	Halfway between flexion and extension
Zygapophyseal joint (facet joint)	Synovial plane joint	Facilitate a multiplanar and multidirectional movement of the spine. Enable lumbar spine extension, lateral flexion, and rotation in the sagittal, transverse and frontal planes.			Full extension	Halfway between flexion and extension

Lumbar Spine Ligaments

Key ligaments	Origin	Insertion	Action/role	Key palpation points
Anterior longitudinal ligament	The anterior portion of the vertebral body at the base of the skull	The anterior portion of the vertebral body at the sacrum	Limits extension of the vertebral column. Reinforces the intervertebral disc.
Posterior longitudinal ligament	The body of C2	Posterior surface of the sacrum	Limits flexion of the vertebral column. Reinforces the intervertebral disc.
Ligamentum flavum: A series of short ligaments that connect the laminae of each vertebra. There are two ligamenta flava at each vertebra. Each ligament is divided into: The medial portion: passes to the back of the next lower lamina and across the gap between the adjacent vertebrae, fusing with the interspinous ligament The lateral portion: passes in front of the facet joint, attaches to the anterior aspect of the inferior and superior articular processes and forms the anterior capsule. Most lateral fibres extend beyond the superior articular process to the pedicle below.	The lower half of the anterior surface of the lamina above	The posterior surface and upper margin of the lamina below	Their high elastin content prevents the ligament from buckling into the spinal canal. Assists with lumbar spine flexion and extension. Compresses the intervertebral discs.
Intertransverse ligaments	Transverse processes of the vertebra above	Transverse processes of the vertebra below	Contributes to the stability of the lumbar spine. Limits lateral flexion.	To palpate the transverse process, position the patient in sitting. The transverse processes of the lumbar spine are most likely located directly in line with the spinous process of the corresponding vertebra. Start by palpating the spinous process of the L1 vertebra and move directly lateral to find the transverse process of L1.
Supraspinous ligament	Tip of spinous processes of the vertebra above from the seventh cervical vertebra to the third or fourth lumbar vertebra	Tip of spinous processes of the vertebra below	Prevents the separation of the spinous processes during forward flexion, thus limiting lumbar spine flexion	You can palpate the spinous processes in the centre of the patient's back. Start with finding T12 by palpating the patient's lowest rib and following your finger along the border of the rib towards the centre of the spine.T12 spinous process is in line with the 12th rib that inserts into the spinal segment. Below T12, you can palpate L1 and continue along the spinous process of L2, L3, and L4.
Interspinous ligament	Spinous processes of the vertebra above between the ligamenta flava anteriorly and the supraspinous ligament posteriorly	Spinous processes of the vertebra below between the ligamenta flava anteriorly and the supraspinous ligament posteriorly	Limits forward flexion of the vertebral column.

Muscles of the Lumbar Spine and Abdominal Wall

The muscles of the back can be grouped according to their function and location.

When grouped according to function, the muscles of the lumbar spine can be organised as follows:

• Flexors: psoas major, psoas minor, internal oblique, external oblique, and rectus abdominis
• Extensors: quadratus lumborum, interspinales lumborum, multifidus, and the erector spinae (iliocostalis, longissimus)
• Lateral flexors: intertransversarii lumborum, psoas major, quadratus lumborum, erector spinae, internal and external obliques
• Rotators: multifidus, internal and external obliques

When grouped according to location, the muscles of the lumbar spine can be divided as follows:

• The intrinsic (deep) group:
  • Superficial layer: erector spinae muscles
  • Middle (deep) layer: multifidus
  • Deepest layer: interspinales lumborum and intertransversarii lumborum
• The abdominal wall group: transverse abdominis, the internal oblique, rectus abdominis, and the external oblique
• The hip group: psoas major, psoas minor, quadratus lumborum

The following tables group muscles according to their function. Please note that when a muscle has multiple functions, it is included in all relevant tables. These tables only list muscle actions associated with the lumbar spine and abdominal wall. If a muscle acts on other joints, these actions are discussed on the relevant page. For example, psoas major is also a major hip flexor - this role is detailed on the Functional Anatomy of the Hip page.

Lumbar Spine Flexors

Muscle	Origin	Insertion	Innervation	Action
Psoas major	Vertebral bodies of T12-L4 Intervertebral discs between T12-L4 Transverse processes of L1-L5 vertebrae	Lesser trochanter of femur	Anterior rami of spinal nerves L1-L3	Contribute to lumbar spine flexion when acting bilaterally and when the insertion point is fixed.
Psoas minor	Vertebral bodies of T12 to L1	Iliopubic eminence	Anterior ramus of spinal nerve L1	Weak trunk flexor. This muscle is absent in 40% of people.
Internal obliques	Thoracolumbar fascia Iliac crest Inguinal ligament	Lower four ribs Linea alba Pubic crest	Lower intercostal nerves Branches of the lumbar plexus: iliohypogastric nerve and ilioinguinal nerve	Bilateral action: lumbar flexion Assist in raising intra-abdominal pressure
External obliques	Ribs five to twelve	Linea alba Pubic tubercle Iliac crest	Intercostal nerves T7- T11 and the T12 subcostal nerve	Bilateral action: lumbar flexion
Rectus abdominis	Pubis, pubic symphysis	Xiphoid process of the sternum Costal cartilage of ribs 5-7	Thoracoabdominal nerves	Lumbar spine flexion Assists in raising intra-abdominal pressure

Lumbar Spine Extensors

Muscle	Origin	Insertion	Innervation	Action
Quadratus lumborum	Iliac crest Iliolumbar ligament	The inferior border of the 12th rib Transverse processes of L1-L4 vertebrae	Subcostal nerve Anterior rami of spinal nerves L1-L4	Bilateral action: trunk extension
Interspinales lumborum	Superior aspects of spinous processes of L2-L5	Inferior aspects of spinous processes of L1-L4	Posterior rami of spinal nerves	Stabilise the lumbar spine Extend the lumbar spine
Multifidus	Sacrum Posterior superior iliac spine The mammillary processes of the lumbar vertebrae Transverse processes of T1-3 Articular processes of C4-C7	Spinous processes of the vertebrae above their origin	Medial branches of the posterior rami of the spinal nerves in the corresponding lumbar region	Extends the lumbar spine Provides core stability
Erector spinae: Iliocostalis lumborum (ICL) Longissimus thoracis (LT) Spinalis (not involved in the lumbar spine motion)	ICL: Iliac crest LT : Lumbar intermuscular aponeurosis, medial part of sacropelvic surface of ilium, posterior sacroiliac ligament	ICL: L1-L4 lumbar transverse processes, angle of 4-12 ribs and thoracolumbar fascia LT: Accessory and transverse processes of vertebrae L1-L5	Dorsal rami of spinal nerves	Bilateral action: lumbar spine extension

Lumbar Spine Lateral Flexors

Muscle	Origin	Insertion	Innervation	Action
Intertransversarii lumborum (ITL): Lateral (ITLL) Medial (ITLM)	ITLL: Transverse and accessory processes of L1-L4 ITLM: Accessory processes of L1-L4	ITLL: Transverse process of the following vertebra ITLM: Mammillary processes of the following vertebra	Anterior rami of spinal nerves	Assists in lateral flexion Stabilises the lumbar spine
Psoas major	Vertebral bodies of T12-L4 Intervertebral discs between T12-L4 Transverse processes of L1-L5 vertebrae	Lesser trochanter of femur	Anterior rami of spinal nerves L1-L3	Lateral flexion of the trunk
Quadratus lumborum	Iliac crest Iliolumbar ligament	The inferior border of the 12th rib Transverse processes of L1-L4 vertebrae	Subcostal nerve Anterior rami of spinal nerves L1-L4	Unilateral action: ipsilateral lumbar spine lateral flexion
Erector spinae: Iliocostalis (IC) Longissimus (L)	ICL: Iliac crest LT : Lumbar intermuscular aponeurosis, medial part of sacropelvic surface of ilium, posterior sacroiliac ligament	ICL: L1-L4 lumbar transverse processes, angle of 4-12 ribs and thoracolumbar fascia LT: Accessory and transverse processes of vertebrae L1-L5	Dorsal rami of spinal nerves	Unilateral action: ipsilateral side flexion and rotation of the vertebral column
Internal oblique	Thoracolumbar fascia Iliac crest Inguinal ligament	Lower four ribs Linea alba Pubic crest	Lower intercostal nerves Branches of the lumbar plexus: iliohypogastric nerve and ilioinguinal nerve.	Unilateral action: lumbar spine lateral flexion and/or rotation to the ipsilateral side Assists in raising intra-abdominal pressure
External oblique	Ribs five to twelve	Linea alba Pubic tubercle Iliac crest	Intercostal nerves T7- T11 and the T12 subcostal nerve	Unilateral action: lumbar spine lateral flexion to the ipsilateral side and rotation to the contralateral side

Lumbar Spine Rotators

Muscle	Origin	Insertion	Innervation	Action
Multifidus	Sacrum Posterior superior iliac spine The mammillary processes of the lumbar vertebrae	Spinous processes of the vertebrae above their origin	Medial branches of posterior rami of spinal nerves in the corresponding lumbar region.	Rotates the lumbar spine away from the side of the body on which they are located Provides core stability
Internal oblique (IO)	Thoracolumbar fascia Iliac crest Inguinal ligament	Lower four ribs Linea alba Pubic crest	Lower intercostal nerves Branches of the lumbar plexus: iliohypogastric nerve and ilioinguinal nerve.	Rotation to the ipsilateral side Works together with opposite EO to rotate the spine - e.g. right external oblique works with left internal oblique to rotate the spine.
External oblique (EO)	Ribs five to twelve	Linea alba Pubic tubercle Iliac crest	Intercostal nerves T7- T11 and the T12 subcostal nerve	Rotation to the contralateral side Works with opposite IO to rotate the spine - e.g. right external oblique works with left internal oblique to rotate the spine.

Innervation of the Lumbar Spine and Abdominal Wall

Nerve	Origin	Branches	Motor fibres	Sensory fibres
Intercostal	Lumbar plexus	Muscular, collateral, lateral cutaneous, anterior cutaneous, and communicating branches	External obliques Internal obliques	Skin over the anterior abdomen
Subcostal	Anterior ramus of the spinal nerve T12	Muscular, cutaneous, communicating and collateral branches	Quadratus lumborum External oblique	The skin of the region under the umbilicus
Iliohypogastric	Lumbar plexus	Lateral cutaneous branch Anterior cutaneous branch	Internal oblique	External abdominal oblique, transversus abdominis, internal abdominal oblique The skin of the suprapubic region
Ilioinguinal	Lumbar plexus	Anterior labial nerves Anterior scrotal nerves	Internal oblique
Dorsal (posterior) rami of spinal nerves	Spinal nerves	Medial Intermediate Lateral	Interspinales lumborum Multifidus Erector spinae	The skin of the back
Ventral (anterior) rami of L1-L4	Spinal nerves		Intertransversarii lumborum Psoas major Psoas minor	The skin of the ventrolateral body walls

Vascular Supply of the Lumbar Spine and Abdominal Wall

Artery	Origin	Branches	Supply
Lumbar artery (LA): Left (LLA) Right (RLA)	Abdominal aorta	Medial Middle Lateral	Skin and muscles of the posterior abdominal wall Joints of the lumbar spine The lumbar portion of the deep back muscles Lower two-thirds of the spinal cord
Internal thoracic artery	Subclavian artery	Anterior collaterals Posterior collaterals Terminal branches	The superior aspect of the abdominal wall
External iliac artery	Common iliac artery	Inferior epigastric, Deep circumflex iliac arteries	Lower abdominal wall

Clinical Relevance

1. Disc herniations are a common low back condition. They occur when the nucleus pulposus displaces from the intervertebral space. According to Yoon et al., when there is imaging confirmation of lumbar disc herniation that is "consistent with clinical findings, and failure to improve after six weeks of conservative care", surgical intervention may be required. You can read more about the management of disc herniation symptoms here.
2. Spinal stenosis is a narrowing of the spinal canal. A range of conditions, such as tumours or bone spurs, can cause spinal stenosis. You can learn about low back pain assessment and prognosis here.
3. Diastasis recti occurs when the linea alba stretches and produces a gap between the two sides of the rectus abdominis muscle (e.g. in pregnancy, obesity). Learn more about this common condition here.
4. Non-specific low back pain is low back pain that is not attributed to a specific pathology. You can learn about lumbar spine assessment by completing An Introduction to Clearing the Lumbar Spine and Considerations for Lumbar Assessment.

Resources

1. Inoue N, Orías AAE, Segami K. Biomechanics of the Lumbar Facet Joint. Spine Surg Relat Res. 2019 Apr 26;4(1):1-7.
2. Kostov S, Dineva S, Kornovski Y, Slavchev S, Ivanova Y, Yordanov A. Vascular Anatomy and Variations of the Anterior Abdominal Wall - Significance in Abdominal Surgery. Prague Med Rep. 2023;124(2):108-142.
3. Clinical Physio. Lumbar Spine Palpation