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 lower back typically includes five vertebrae in the lumbar region and supports much of the upper body's weight. Intervertebral discs maintain the spaces between the vertebrae, which 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. It contains the skin, fascia, and muscle, which provide coverage for the organs, generates inter-abdominal pressure, and move the vertebral column. This article discusses the key anatomical structures of the lumbar vertebra 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 a 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

L1-L5 are known as lumbar vertebrae. However, 10–17.4% of adults present with a spinal abnormality of having the 6th lumbar vertebra. This abnormality may influence the saggital parameters, including a diminished sacral tilt and more vertical sacrum. The presence of additional lumbar vertebrae can be associated with a decrease in the number of thoracic vertebrae (individuals with eleven thoracic vertebrae).

The lumbar vertebrae are located between the 12th 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 from side to side than from 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
  • Becoming shorter, broader and more lateral from L1 - L5
• Two laminae
  • Flat and broad
  • Two pedicles and two laminae form the vertebral arch
  • Centrally connects to a spinous process
• Spinous process
  • Projects perpendicularly from the body
  • Short and sturdy
  • Describe as a "hatchet-shaped"
  • Point of muscle and ligament attachment
• Two transverse processes
  • Located on the posterior surface
  • Project laterally on each side of the vertebra
  • Long and slender
  • Points of muscle and ligaments attachment
  • L1-L3 projecting horizontally
  • L4-L5 projecting upward
• Four articular processes
  • Two superior articular facets and two inferior articular facets
  • The point where two facets are joining is called the zygapophyseal joints

Lumbar Lordosis

The natural curve for the lumbar spine is lordosis. It defines lumbar spine alignment in the saggital plane. However, the pelvis is "the cornerstone of spinal sagittal alignment". The curve of the sacrum determines the lumbar curve.

The lumbar spine sagittal alignment offers two main 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 the lumbar spine alignment must include the analysis of the lower limbs alignment. The knee flexion angle measurement and pelvic shift results can help identify the compensatory mechanisms resulting from the sagittal plane malalignment.

Abdominal Wall Structure

The abdominal wall has skeletal support made of the vertebral column and lower ribs posteriorly and connects to the thoracic cage at the superior end and pelvic bones inferiorly. It is defined by the following landmarks: xiphoid process and costal cartilages of the 7th–10th ribs outline the superior border, the umbilicus in the middle, and inguinal ligament, pubic crest and pubic symphysis define the inferior border.

You can read more about pelvis anatomy here.

Bones and Articulations of the Lumbar Spine

The lumbar spine has two types of joints located between each lumbar vertebra: the intervertebral disc joint and the zygapophyseal joint or the facet joint. The intervertebral disc joint is formed between the vertebral bodies and is separated by the intervertebral disc. The zygapophyseal joints are formed between the articular processes of each lumbar vertebra.

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 spinal processes will determine the location of each segment. You can palpate the spinal 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 spinal process is in line with the 12th rib that inserts into the spinal segment. Below T12, you can palpate L1 and continue along the spinal process of L2, L3, and L4. When starting your palpation from the distal end of the lumbar spine, you must consider the following caution: the spinal process of L4 "is usually determined by the position of the highest point on both sides of PSIS", and the accuracy of palpation is only 36%.
Articular processes	Zygapophyseal joint (Facet joint)	Located on the left and right sides of the spine Links one vertical segment to the other. For example, a 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. The primary role includes the stabilization of the spinal motion segment.C Contribute to axial compressive load transmission on the spine	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 spinal process, you will be on the muscles overlying the facet joint.

Lumbar Spine Kinematics

The lumbar spine is considered a three-joint complex or the articular triad. The zygapophyseal joint or facet joints and intervertebral disk joints belong to the spinal motion segment, which, when worked together, allow spinal motion and protect the spine from excessive movements.

The following is the characteristic of the spinal motion segment:

• The intervertebral disk joints transmit mostly vertical compressive loads
• The facet joints guide and stabilise the motion segment
• In the 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 All lumbar intervertebral disc joints and zygapophyseal joints combined together: Flexion/Extension Lateral flexion Rotation	All lumbar intervertebral disc joints and zygapophyseal joints combined 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 saggital transverse and frontal planes.	Flexion/Extension Lateral flexion Rotation	All lumbar intervertebral disc joints and zygapophyseal joints combined together: Flexion:60 degrees Extension 25 degrees Lateral flexion 20-30 degrees Rotation 1-5 degrees	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 the C2	Posterior surface of the sacrum	Limits flexion of the vertebral column Reinforces the intervertebral disc
Ligamentum flavum: Series of short ligaments that connects 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	High elastin content, which 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 Provides the opposite side resistance during lateral flexion	To palpate the transverse process, the patient is sitting. The transverse processes of the lumbar spine are most likely located directly in line with the spinous process of the corresponding vertebra. Start with palpating the spinous process of the L1 vertebra and move directly lateral to find the spinal 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 according to their location.

Based on the function the muscles of the lumbar spine are the following:

• 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

According to the location, the muscles of the lumbar spine can be divided as follow:

• 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

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	Bilateral action and when the insertion point is fixed: contribute to lumbar spine flexion.
Psoas minor	Vertebral bodies of T12 to L1	Iliopubic eminence	Anterior ramus of spinal nerve L1	Weak trunk flexor. It can be 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 Assists 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 The fifth to the seventh costal cartilage	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 posterior rami of spinal nerves in the corresponding lumbar region	Extends the lumbar spine Provides core stability
Erector spinae: Iliocostalis lumborum (ICL) Longissimus thoracis (LT)	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 same side Work together with EO: right external and left internal oblique 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 opposite side Work together with IO: right external and left internal oblique 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	Ventrolateral body surface Structures in the body wall

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 common low back conditions. They are defined by the nucleus pulposus displacing from intervertebral space. According to Yoon et al., imaging confirmation of lumbar disc herniation, "consistent with clinical findings, and failure to improve after six weeks of conservative care", may require surgical intervention. You can read more about disc herniations symptoms management here.
2. Spinal stenosis is a narrowing of the spinal canal. Spinal stenosis can also result from other factors, such as tumours or bone spurs. You can learn about low back pain assessment and prognosis by taking this course.
3. Diastasis recti is an extremely frequent condition where the linea alba stretches and produces a gap between the two sides of the rectus abdominis muscle. Learn more about diastasis recti here.

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