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ABDOMINAL MUSCLES
ABDOMINAL WALL
24. 04. 2014
Kaan Yücel
M.D., Ph.D.
http://yeditepeanatomy1.org
Dr. Kaan Yücel
http://yeditepeanatomy1.org
Abdominal muscles & Abdominal wall
Although the abdominal wall is continuous, it is subdivided into the anterior wall, right and left lateral walls,
and posterior wall for descriptive purposes. The wall is musculoaponeurotic, except for the posterior wall, which
includes the lumbar region of the vertebral column. The boundary between the anterior and the lateral walls is
indefinite, therefore the term anterolateral abdominal wall is often used. The anterolateral abdominal wall is
bounded superiorly by the cartilages of the 7th-10th ribs and the xiphoid process of the sternum, and inferiorly by
the inguinal ligament and the superior margins of the anterolateral aspects of the pelvic girdle (iliac crests, pubic
crests, and pubic symphysis).
The superficial fascia of the abdominal wall (subcutaneous tissue of abdomen) is a layer of fatty connective
tissue and continuous with, the superficial fascia throughout other regions of the body. Below the umbilicus, it
forms two layers: a superficial fatty layer and a deeper membranous layer. The superficial fatty layer of superficial
fascia (Camper's fascia) contains fat and varies in thickness. The deeper membranous layer of superficial fascia
(Scarpa's fascia) is thin and membranous, and contains little or no fat. Inferiorly, it continues into the thigh, but just
below the inguinal ligament, it fuses with the deep fascia of the thigh (the fascia lata).
There are five muscles in the anterolateral group of abdominal wall muscles:
three flat muscles whose fibers begin posterolaterally, pass anteriorly, and are replaced by an aponeurosis as
the muscle continues towards the midline: external oblique, internal oblique, and transversus abdominis muscles;
two vertical muscles, near the midline, which are enclosed within a tendinous sheath formed by the
aponeuroses of the flat muscles-the rectus abdominis and pyramidalis muscles.
The most superficial of the three flat muscles in the anterolateral group of abdominal wall muscles is the
external oblique, which is immediately deep to the superficial fascia. Deep to the external oblique muscle is the
internal oblique muscle, which is the second of the three flat muscles. Deep to the internal oblique muscle is the
transversus abdominis muscle, so named because of the direction of most of its muscle fibers.
Functions of the muscles of the anterolateral abdominal wall:
-Form a strong expandable support for the anterolateral abdominal wall.
-Support the abdominal viscera and protect them from most injuries.
-Compress the abdominal contents to maintain or increase the intra-abdominal pressure and, in so doing, oppose the
diaphragm (increased intra-abdominal pressure facilitates expulsion).
-Move the trunk and help to maintain posture.
External oblique is innervated by T7-T11 spinal nerves and subcostal nerve. Internal oblique is innervated by the
anterior rami of T6-T12 spinal nerves) and first lumbar nerves.Transversus abdominis is innervated by the anterior
rami of T6-T12 spinal nerves) and first lumbar nerves.Rectus abdominis is innervated by the anterior rami of T6T12 spinal nerves.
The main paired muscles in the posterior abdominal wall are the:
Psoas major: passing inferolaterally.
Iliacus: lying along the lateral sides of the inferior part of the psoas major.
Quadratus lumborum: lying adjacent to the transverse processes of the lumbar vertebrae and lateral to superior parts
of the psoas major.
The psoas passes inferolaterally, deep to the inguinal ligament to reach the lesser trochanter of the femur. The
lumbar plexus of nerves is embedded in the posterior part of the psoas, anterior to the lumbar transverse processes.
The psoas major muscle flexes the thigh at the hip joint when the trunk is stabilized and flexes the trunk against
gravity when the body is supine. It is innervated by anterior rami of nerves L1 to L3.
Associated with the psoas major muscle is the psoas minor muscle, which is sometimes absent.It lies on the surface
of the psoas major when present. It is a weak flexor of the lumbar vertebral column and is innervated by the anterior
ramus of nerve L1.
Laterally, the quadratus lumborum muscles fill the space between ribs XII and the iliac crest on both sides of the
vertebral column. They are overlapped medially by the psoas major muscles. The quadratus lumborum muscles
depress and stabilize the twelfth ribs and contribute to lateral bending of the trunk. They are innervated by anterior
rami of T12 and L1 to L4 spinal nerves.
Inferiorly, an iliacus muscle fills the iliac fossa on each side. It joins with the psoas major muscle, and attaches to
the lesser trochanter of the femur. As they pass into the thigh, these combined muscles are referred to as the
iliopsoas muscle.Together the psoas and iliacus form the iliopsoas, the chief flexor of the thigh. It is also a stabilizer
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of the hip joint and helps maintain the erect posture at this joint. The psoas and iliacus share in hip flexion; however,
only the psoas can produce movement (flexion or lateral bending) of the lumbar vertebral column. It is innervated
by branches of the femoral nerve.
Dr. Kaan Yücel
http://yeditepeanatomy1.org
Abdominal muscles & Abdominal wall
1. ABDOMINAL WALL
The abdomen is the part of the trunk between the thorax and the pelvis. It is a flexible, dynamic
container, housing most of the organs of the alimentary system and part of the urogenital system. Containment
of the abdominal organs and their contents is provided by musculoaponeurotic walls anterolaterally, the
diaphragm superiorly, and the muscles of the pelvis inferiorly.
The anterolateral musculoaponeurotic walls are suspended between and supported by two bony rings
(the inferior margin of the thoracic skeleton superiorly and pelvic girdle inferiorly) linked by a semirigid lumbar
vertebral column in the posterior abdominal wall. Interposed between the more rigid thorax and pelvis, this
arrangement enables the abdomen to enclose and protect its contents while providing the flexibility required
by respiration, posture, and locomotion.
Through voluntary or reflexive contraction, its muscular roof, anterolateral walls, and floor can raise internal
(intraabdominal) pressure to aid expulsion from the abdominopelvic cavity or from the adjacent thoracic cavity,
expulsion of air from the thoracic cavity (lungs and bronchi) or of fluid (e.g., urine or vomitus), flatus, feces, or
fetuses from the abdominopelvic cavity.
The abdominal wall covers a large area. It is bounded superiorly by the xiphoid process and costal margins,
posteriorly by the vertebral column, and inferiorly by the upper parts of the pelvic bones. Its layers consist of
skin, superficial fascia (subcutaneous tissue), muscles and their associated deep fascias, extraperitoneal fascia,
and parietal peritoneum.
Although the abdominal wall is continuous, it is subdivided into the anterior wall, right and left lateral walls,
and posterior wall for descriptive purposes. The wall is musculoaponeurotic, except for the posterior wall,
which includes the lumbar region of the vertebral column. The boundary between the anterior and the lateral
walls is indefinite, therefore the term anterolateral abdominal wall is often used. Some structures, such as
muscles and cutaneous nerves, are in both the anterior and lateral walls. The anterolateral abdominal wall
extends from the thoracic cage to the pelvis. The anterolateral abdominal wall is bounded superiorly by the
cartilages of the 7th-10th ribs and the xiphoid process of the sternum, and inferiorly by the inguinal ligament
and the superior margins of the anterolateral aspects of the pelvic girdle (iliac crests, pubic crests, and pubic
symphysis).
Superficial fascia
The superficial fascia of the abdominal wall (subcutaneous tissue of abdomen) is a layer of fatty connective
tissue. It is usually a single layer similar to, and continuous with, the superficial fascia throughout other regions
of the body. However, in the lower region of the anterior part of the abdominal wall, below the umbilicus, it
forms two layers: a superficial fatty layer and a deeper membranous layer.
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Abdominal muscles & Abdominal wall
Superficial layer
The superficial fatty layer of superficial fascia (Camper's fascia) contains fat and varies in thickness. It is
continuous over the inguinal ligament with the superficial fascia of the thigh and with a similar layer in the
perineum. In men, this superficial layer continues over the penis and, after losing its fat and fusing with the
deeper layer of superficial fascia, continues into the scrotum where it forms a specialized fascial layer
containing smooth muscle fibers (the dartos fascia). In women, this superficial layer retains some fat and is a
component of the labia majora.
The deeper membranous layer of superficial fascia (Scarpa's fascia) is thin and membranous, and contains
little or no fat. Inferiorly, it continues into the thigh, but just below the inguinal ligament, it fuses with the deep
fascia of the thigh (the fascia lata). In the midline, it is firmly attached to the linea alba and the symphysis pubis.
It continues into the anterior part of the perineum where it is firmly attached to the ischiopubic rami and to the
posterior margin of the perineal membrane. Here, it is referred to as the superficial perineal fascia (Colles'
fascia).
The deep fascia of the abdominal wall is continous with the fascia lata inferiorly in the thigh, and is also
continous with the fascia of the perineum posteriorly and the deep fascia covering the adductor muscles of the
thigh laterally.
There are five muscles in the anterolateral group of abdominal wall muscles:
three flat muscles whose fibers begin posterolaterally, pass anteriorly, and are replaced by an aponeurosis as
the muscle continues towards the midline: external oblique, internal oblique, and transversus abdominis
muscles;
two vertical muscles, near the midline, which are enclosed within a tendinous sheath formed by the
aponeuroses of the flat muscles-the rectus abdominis and pyramidalis muscles.
Each of these five muscles has specific actions, but together the muscles are critical for the maintenance of
many normal physiological functions. By their positioning, they form a firm, but flexible, wall that keeps the
abdominal viscera within the abdominal cavity, protects the viscera from injury, and helps maintain the position
of the viscera in the erect posture against the action of gravity.
In addition, contraction of these muscles assists in both quiet and forced expiration by pushing the viscera
upward (which helps push the relaxed diaphragm further into the thoracic cavity) and in coughing and
vomiting.
All these muscles are also involved in any action that increases intra-abdominal pressure, including
parturition (childbirth), micturition (urination), and defecation (expulsion of feces from the rectum).
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Dr. Kaan Yücel
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Abdominal muscles & Abdominal wall
Flat muscles
The most superficial of the three flat muscles in the anterolateral group of abdominal wall muscles is the
external oblique, which is immediately deep to the superficial fascia. Its laterally placed muscle fibers pass in an
inferomedial direction, while its large aponeurotic component covers the anterior part of the abdominal wall to
the midline. Approaching the midline, the aponeuroses are entwined, forming the linea alba, which extends
from the xiphoid process to the pubic symphysis.
Associated ligaments
The lower border of the external oblique aponeurosis forms the inguinal ligament (Poupart’s ligament) on
each side. This thickened reinforced free edge of the external oblique aponeurosis passes between the anterior
superior iliac spine laterally and the pubic tubercle medially. It folds under itself forming a trough, which plays
an important role in the formation of the inguinal canal.
Several other ligaments are also formed from extensions of the fibers at the medial end of the inguinal
ligament: lacunar ligament is a crescent-shaped extension of fibers at the medial end of the inguinal ligament
that pass backward to attach to the pecten pubis on the superior ramus of the pubic bone; additional fibers
extend from the lacunar ligament along the pecten pubis of the pelvic brim to form the pectineal (Cooper's)
ligament.
Deep to the external oblique muscle is the internal oblique muscle, which is the second of the three flat
muscles. This muscle is smaller and thinner than the external oblique, with most of its muscle fibers passing in a
superomedial direction. Its lateral muscular components end anteriorly as an aponeurosis that blends into the
linea alba at the midline.
Deep to the internal oblique muscle is the transversus abdominis muscle, so named because of the
direction of most of its muscle fibers. It ends in an anterior aponeurosis, which blends with the linea alba at the
midline.
Transversalis fascia
Each of the three flat muscles is covered on its anterior and posterior surfaces by a layer of deep fascia.
In general, these layers are unremarkable except for the layer deep to the transversus abdominis muscle (the
transversalis fascia), which is better developed. The transversalis fascia is a continuous layer of deep fascia that
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Abdominal muscles & Abdominal wall
lines the abdominal cavity and continues into the pelvic cavity. It crosses the midline anteriorly, associating with
the transversalis fascia of the opposite side, and is continuous with the fascia on the inferior surface of the
diaphragm. It is continuous posteriorly with the deep fascia covering the muscles of the posterior abdominal
wall and attaches to the thoracolumbar fascia.
There is therefore a continuous layer of deep fascia surrounding the abdominal cavity that is thick in some
areas, thin in others, attached or free, and participates in the formation of specialized structures.
Vertical muscles
The two vertical muscles in the anterolateral group of abdominal wall muscles are the large rectus
abdominis and the small pyramidalis.
The rectus abdominis is a long, flat muscle and extends the length of the anterior abdominal wall. It is a
paired muscle, separated in the midline by the linea alba, and it widens and thins as it ascends from the pubic
symphysis to the costal margin. Along its course, it is intersected by three or four transverse fibrous bands or
tendinous intersections. These are easily visible on individuals with a well-developed rectus abdominis (Kaan’s
note: “Not me”).
The second vertical muscle is the pyramidalis. This small, triangular muscle, which may be absent, is anterior
to the rectus abdominis, has its base on the pubis, and its apex is attached superiorly and medially to the linea
alba. It is innervated by the anterior ramus of T12 (twelth thoracic spinal nerve) .
The rectus sheath is separated from its fellow on the opposite side by a fibrous band called the linea alba.
This extends from the xiphoid process down to the symphysis pubis and is formed by the fusion of the
aponeuroses of the lateral muscles of the two sides. Wider above the umbilicus, it narrows down below the
umbilicus to be attached to the symphysis pubis.
The posterior wall of the rectus sheath is not attached to the rectus abdominis muscle. The anterior wall is
firmly attached to it by the muscle's tendinous intersections.
Rectus sheath
The rectus abdominis and pyramidalis muscles are enclosed in an aponeurotic tendinous sheath (rectus
sheath) formed by a unique layering of the aponeuroses of the external and internal oblique, and transversus
abdominis muscles. The rectus sheath completely encloses the upper three-quarters of the rectus abdominis
and covers the anterior surface of the lower one-quarter of the muscle. At a point midway between the
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umbilicus and the pubic symphysis, corresponding to the beginning of the lower one-quarter of the rectus
abdominis muscle, all of the aponeuroses move anterior to the rectus muscle. There is no posterior wall of the
rectus sheath and the anterior wall of the sheath consists of the aponeuroses of the external oblique, the
internal oblique, and the transversus abdominis muscles. As no sheath covers the posterior surface of the
lower quarter of the rectus abdominis muscle, the muscle at this point is in direct contact with the transversalis
fascia. Marking this point of transition is an arch of fibers (the arcuate line).
The formation of the rectus sheath surrounding the upper three-quarters of the rectus abdominis muscle
has the following pattern:
 anterior wall consists of the aponeurosis of the external oblique and half of the aponeurosis of the internal
oblique, which splits at the lateral margin of the rectus abdominis;
 posterior wall of the rectus sheath consists of the other half of the aponeurosis of the internal oblique and
the aponeurosis of the transversus abdominis.
FUNCTIONS AND ACTIONS OF ANTEROLATERAL ABDOMINAL MUSCLES
The muscles of the anterolateral abdominal wall:
Form a strong expandable support for the anterolateral abdominal wall.
Support the abdominal viscera and protect them from most injuries.
Compress the abdominal contents to maintain or increase the intra-abdominal pressure and, in so doing,
oppose the diaphragm (increased intra-abdominal pressure facilitates expulsion).
Move the trunk and help to maintain posture.
The oblique and transverse muscles, acting together bilaterally, form a muscular girdle that exerts firm
pressure on the abdominal viscera. The rectus abdominis participates little, if at all, in this action. Compressing
the abdominal viscera and increasing intra-abdominal pressure elevates the relaxed diaphragm to expel air
during respiration and more forcibly for coughing, sneezing, nose blowing, voluntary eructation (burping), and
yelling or screaming. When the diaphragm contracts during inspiration, the anterolateral abdominal wall
expands as its muscles relax to make room for the organs, such as the liver, that are pushed inferiorly. The
combined actions of the anterolateral muscles also produce the force required for defecation (discharge of
feces), micturition (urination), vomiting, and parturition (childbirth). Increased intra-abdominal (and
intrathoracic) pressure is also involved in heavy lifting, the resulting force sometimes producing a hernia.
The anterolateral abdominal muscles are also involved in movements of the trunk at the lumbar vertebrae
and in controlling the tilt of the pelvis when standing for maintenance of posture (resisting lumbar lordosis).
Consequently, strengthening the anterolateral abdominal wall musculature improves standing and sitting
posture. The rectus abdominis is a powerful flexor of the thoracic and especially lumbar regions of the vertebral
column, pulling the anterior costal margin and pubic crest toward each other. The oblique abdominal muscles
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Abdominal muscles & Abdominal wall
also assist in movements of the trunk, especially lateral flexion and rotation of the lumbar and lower thoracic
vertebral column. The transversus abdominis probably has no appreciable effect on the vertebral column.
The main paired muscles in the posterior abdominal wall are the:
Psoas major: passing inferolaterally.
Iliacus: lying along the lateral sides of the inferior part of the psoas major.
Quadratus lumborum: lying adjacent to the transverse processes of the lumbar vertebrae and lateral to
superior parts of the psoas major.
Muscles forming the medial, lateral, inferior, and superior boundaries of the posterior abdominal region fill
in the bony framework of the posterior abdominal wall.
The long, thick, fusiform psoas major lies lateral to the lumbar vertebrae. Psoas is a Greek word meaning
“muscle of the loin.” (Butchers refer to the psoas of animals as the tenderloin.) The psoas major passes
inferolaterally, deep to the inguinal ligament to reach the lesser trochanter of the femur. The lumbar plexus of
nerves is embedded in the posterior part of the psoas, anterior to the lumbar transverse processes.
Medially, the psoas major muscles cover the anterolateral surface of the bodies of the lumbar vertebrae,
filling in the space between the vertebral bodies and the transverse processes. Passing inferiorly along the
pelvic brim, each muscle continues into the anterior thigh, under the inguinal ligament, to attach to the lesser
trochanter of the femur.
The psoas major muscle flexes the thigh at the hip joint when the trunk is stabilized and flexes the trunk
against gravity when the body is supine. It is innervated by anterior rami of nerves L1 to L3.
Associated with the psoas major muscle is the psoas minor muscle, which is sometimes absent. Lying on the
surface of the psoas major when present, this slender muscle arises from vertebrae TXII and LI and the
intervening intervertebral disc; its long tendon inserts into the pectineal line of the pelvic brim and the iliopubic
eminence. The psoas minor is a weak flexor of the lumbar vertebral column and is innervated by the anterior
ramus of nerve L1.
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Laterally, the quadratus lumborum muscles fill the space between ribs XII and the iliac crest on both sides of
the vertebral column. They are overlapped medially by the psoas major muscles; along their lateral borders are
the transversus abdominis muscles. The quadrilateral quadratus lumborum forms a thick muscular sheet in the
posterior abdominal wall. It lies adjacent to the lumbar transverse processes and is broader inferiorly. Branches
of the lumbar plexus run inferiorly on the anterior surface of this muscle.
Each quadratus lumborum muscle arises from the transverse processes of L5, the iliolumbar ligament, and
the adjoining part of the iliac crest. The muscle attaches superiorly to the transverse process of the first four
lumbar vertebrae and the inferior border of rib XII.
The quadratus lumborum muscles depress and stabilize the twelfth ribs and contribute to lateral bending of
the trunk. Acting together, the muscles may extend the lumbar part of the vertebral column. They are
innervated by anterior rami of T12 and L1 to L4 spinal nerves.
Inferiorly, an iliacus muscle fills the iliac fossa on each side. From this expansive origin covering the iliac
fossa, the muscle passes inferiorly, joins with the psoas major muscle, and attaches to the lesser trochanter of
the femur. As they pass into the thigh, these combined muscles are referred to as the iliopsoas muscle.
The iliacus is a large triangular muscle that lies along the lateral side of the inferior part of the psoas major.
Most of its fibers join the tendon of the psoas major. Together the psoas and iliacus form the iliopsoas, the
chief flexor of the thigh. It is also a stabilizer of the hip joint and helps maintain the erect posture at this joint.
The psoas and iliacus share in hip flexion; however, only the psoas can produce movement (flexion or lateral
bending) of the lumbar vertebral column. It is innervated by branches of the femoral nerve.
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Abdominal muscles & Abdominal wall
Table 1 . Muscles of the anterolateral abdominal wall
Muscle
External oblique
Origin
Insertion
Innervation
Main Actiona
External surfaces of 5th-
Linea alba, pubic
T7-T11 spinal
Compresses and
12th ribs
tubercle, and anterior
nerves and
supports abdominal
half of iliac crest
subcostal nerve
viscera,b flexes and
rotates trunk
Internal oblique
Thoracolumbar fascia,
Inferior borders of 10th-
Anterior rami of
Compresses and
anterior 2/3 of iliac
12th ribs, linea alba, and
T6-T12 spinal
supports abdominal
crest, and connective
pecten pubis via conjoint
nerves) and first
viscera, flexes and
tissue deep to lateral 1/3
tendon
lumbar nerves
rotates trunk
of inguinal ligament
Transversus
Internal surfaces of 7th-
Linea alba with
Anterior rami of
Compresses and
abdominis
12th costal cartilages,
aponeurosis of internal
T6-T12 spinal
supports abdominal
thoracolumbar fascia,
oblique, pubic crest, and
nerves) and first
viscerab
iliac crest, and
pecten pubis via conjoint
lumbar nerves
connective tissue deep
tendon
to lateral 1/3 of inguinal
ligament
Rectus abdominis Pubic symphysis and
pubic crest
Xiphoid process and 5th-
Anterior rami of
Compress abdominal
7th costal cartilages
T6-T12 spinal
contents; flex
nerves
vertebral column;
tense abdominal wall
Pyramidalis *
Front of pubis and pubic
symphysis
Into linea alba
Anterior ramus of
Tenses the linea alba
T12
* Approximately 80% of people have an insignificant muscle, the pyramidalis,which is located in the rectus sheath anterior to the most
inferior part of the rectus abdominis. It extends from the pubic crest of the hip bone to the linea alba. This small muscle draws down on
the linea alba.
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Table 2 . Muscles of the posterior abdominal wall
Muscle
Origin
Insertion
Innervation
Main Action
Psoas major
Transverse processes of
By a strong tendon
Anterior rami of lumbar
Acting inferiorly with
lumbar vertebrae;
to lesser
nerves L1, L2, L3
iliacus, it flexes thigh;
bodies of T12-L5
trochanter of
acting superiorly it
vertebrae and
femur
flexes vertebral
intervening
column laterally; it is
intervertebral discs
used to balance the
trunk; when sitting it
acts inferiorly with
iliacus to flex trunk
Psoas minor
TXII and LI vertebrae
Pectineal line of
Anterior rami of L1
Weak flexion of
the pelvic brim
lumbar vertebral
and iliopubic
column
eminence
Iliacus
Superior 2/3 of iliac
Lesser trochanter
Femoral nerve
Flexes thigh and
fossa, ala of sacrum, and
of femur and shaft
stabilizes hip joint;
anterior sacroiliac
inferior to it, and
acts with psoas major
ligaments
to psoas major
tendon
Quadratus
Transverse process of L5,
Transverse process Anterior branches of T12
Depress and stabilize
lumborum
Iliolumbar ligament, iliac
of the first four
rib XII during
crest
lumbar vertebrae
inspiration and some
and the inferior
lateral flexion of
border of rib XII.
trunk
and L1-L4 nerves
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