Video: Muscles of the abdominal wall

Have you ever held a plank position and found that time moves inexplicably slower than normal while you sweat out each second? Whether you have endured planking to have six-pack abs or you recall struggling during core exercises in gym class, you have surely felt your abdominal muscles engaging, contracting, and yes, burning while doing so. Ultimately, the muscles of our abdominal wall serve an important purpose and their arrangement and anatomical relationships might be more intricate than you'd expect. With that said, let's sit up straight and learn about the muscles of the abdominal wall.

Where exactly do we start? We'll begin by reviewing the names and arrangement of all the muscles of the abdominal wall. Next, we'll take a look at the attachment sites and innervation patterns for each muscle. As we venture through this tutorial, we will gain an understanding for the anatomical relationships between the muscles of the abdominal wall. Finally, we will finish off with a look at clinical notes surrounding this group of muscles to help consolidate our knowledge.

Before we set off, let's briefly discuss some overarching details that will help carve out our understanding of the muscles of the abdominal wall.

Functionally, the muscles of the abdominal wall provide coverage, stability and flexible movement to the abdominal cavity whilst also protecting the abdominal viscera from injury. Furthermore, according to their location, these muscles can be divided into two compartments – the anterolateral and the posterior wall muscles. So, are you feeling absolutely ready to master these abdominal wall muscles? Fantastic! Let's begin with the muscles of the anterolateral abdominal wall.

There are five bilaterally paired muscles of the anterolateral abdominal wall, and as the name suggests, these muscles are located on the anterior and lateral surfaces of the abdominal cavity. Muscles of the anterolateral abdominal wall can be divided into two groups according to their fiber direction. There are three flat muscles and two vertical muscles. The three flat muscles consist of the external oblique, internal oblique, and transversus abdominis muscles. These muscles function to flex and rotate the trunk as well as compress the abdominal contents thereby increasing intra-abdominal pressure and supporting the process of urination and defecation. The increased intra-abdominal pressure caused by the contraction of the abdominal wall muscles also assists in giving birth. Forced expiration using the abdominal muscles also helps expel materials from the airways as in coughing or sneezing.

The two vertical muscles are the rectus abdominis, better known as the abs muscle, and the small pyramidalis muscle. These muscles are enveloped by the rectus sheath and are related to the linea alba. Both of these structures are formed by the aponeurosis of the flat muscles of the anterolateral abdominal wall.

Let's start off with taking a look at the three flat muscles. First up, the most superficial muscle of the lateral abdominal wall is the external oblique. This flat muscle arises laterally from the external surface of ribs 5 to 12 and its fibers span inferomedially. An easy way to picture the direction in which these muscle fibers lie is to place your hands into the pockets of your trousers. The inferomedial position that your fingers are oriented in is similar to the direction in which the muscle fibers of the external oblique muscle lie.

The external oblique muscle extends into a broad aponeurosis anteriorly. This broad aponeurosis is a tendinous sheath made of connective tissue that inserts at multiple sites including the linea alba at the midline and inferiorly at the iliac crest and pubic tubercle. The inferior edge of the aponeurosis of the external oblique muscle is thickened and forms a fibrous band which extends from the anterior superior iliac spine to the pubic tubercle. This fibrous band is known as the inguinal ligament. Important neurovascular structures pass under the inguinal ligament to supply the lower limb, therefore, the inguinal ligament acts as a retinaculum protecting and holding these structures in place.

Somewhat intuitively based on its location, the external oblique muscle is innervated by the lower intercostal nerves, subcostal nerve, and sometimes, the iliohypogastric nerve. During bilateral contraction, this muscle facilitates trunk flexion and compression of abdominal viscera whilst also contributing to expiration. Unilateral contraction of this muscle contributes to ipsilateral flexion and contralateral rotation of the trunk.

As the name suggests, our next muscle – the internal oblique – is situated just deep to the external oblique. It originates from the thoracolumbar fascia posteriorly, the iliac crest laterally, and inguinal ligament inferiorly. Note that some sources include the iliopectineal arch as one of the origin points instead of the inguinal ligament which is found right next to it. Most of the internal oblique’s muscle fibers are oriented superomedially and lie perpendicular to the muscle fibers of the external oblique muscle. This muscle inserts superiorly onto the inferior borders of ribs 10 to 12, the linea alba at the midline, and inferiorly at the pubic crest and pectineal line of the pubic bone.

The internal oblique muscle is supplied by the lower intercostal nerves, the subcostal nerve as well as the iliohypogastric and ilioinguinal nerves which are both branches of the lumbar plexus. Similar to the external oblique, the internal oblique muscle also facilitates flexion of the trunk, compression of abdominal viscera, and aids in expiration during bilateral contraction of the muscle. During unilateral contraction, the muscle contributes ipsilateral flexion and ipsilateral rotation of the trunk.

Living up to its name, the transversus abdominis muscle fibers transverse across the abdomen. As the deepest muscle of the lateral abdominal wall, it arises from the inner surface of the costal cartilages of ribs 7 to 12, the thoracolumbar fascia posteriorly, the iliac crest, and the inguinal ligament. As we have seen with the internal oblique, in some sources you might see the iliopectineal arch listed as one of the origin points.

The transversus abdominis extends horizontally to insert at the linea alba, pubic crest, and pectineal line of the pubic bone and the aponeurosis of the internal abdominal oblique muscle. Conveniently, the innervation pattern for this muscle is analogous to the internal oblique thus it's supplied by the lower intercostal nerves, the subcostal nerve and the iliohypogastric and inguinal nerves. Similar to the external and internal oblique muscles during bilateral contraction, the transversus abdominis muscle also aids in compressing and supporting abdominal viscera whilst facilitating expiration. While during unilateral contraction, it is involved in ipsilateral rotation of the trunk.

The inferior fibers of the internal oblique and transverse abdominis muscles form a sheath of connective tissue known as the conjoint tendon. The conjoint tendon attaches to the pubic crest and extends to insert along the pectineal line of the pubis. It is situated just posterior to the superficial inguinal ring and functions to strengthen the medial portion of the posterior wall of the inguinal canal.

Also formed by contributions from the internal oblique and transverse abdominis muscles of the abdominal wall is the cremaster muscle. This muscle consists of a lateral portion that arises from the inferomedial portion of the internal oblique and transverse abdominis muscles. The cremaster muscle surrounds the spermatic cord in males and the round ligament of the uterus in females and contributes to the formation of cremasteric fascia.

Traveling through the inferior portions of the anterior abdominal wall muscles is a narrow oblique passageway known as the inguinal canal. It surrounds the spermatic cord in males, the round ligament of the uterus in females, and the ilioinguinal nerve in both sexes. The inguinal canal has both superficial and deep openings which are known as the superficial and deep inguinal rings of the inguinal canal.

The deep inguinal ring is a hiatus in the transversalis fascia and is located approximately midway between the anterior superior iliac spine and the pubic symphysis. The superficial inguinal ring is an opening in the aponeurosis of the external abdominal oblique muscle. It marks the end of the inguinal canal. Fibers from the aponeurosis of the external oblique muscle extend downwards from the ring over the spermatic cord.

All right, who's ready to move on to the vertical muscles and learn about those six-pack abs? Before we start chiseling away, let's look at the rectus abdominis muscles.

The rectus abdominis is a pair of long, vertically-oriented muscles separated by the linea alba that itself is formed by the interlacing aponeurosis of the three flat abdominal muscles. This muscle is intersected by three or four transverse fibrous bands known as tendinous intersections. The shape of these segments is clearly demarcated when the rectus abdominis is well-defined or in persons with low body fat percentage resulting in the commonly known six-pack appearance.

The rectus abdominis extends the length of the anterior abdominal wall. It arises from the pubic crest and pubic symphysis and interlaces with tendinous fibers of the muscle on the contralateral side. Traversing superiorly, the rectus abdominis muscles then insert onto the xiphoid process of the sternum and the costal cartilages of ribs 5 to 7.

Functioning to flex the trunk and accomplish those laborious sit-ups, the rectus abdominis is supplied by the intercostal nerves and the subcostal nerve. As already mentioned, the rectus abdominis muscles function to flex the trunk, stabilize the pelvis, and compress and support the contents of the abdomen. In addition to all of that, this muscle aids in expiration. In closing the rectus abdominis, there is an aponeurotic tendinous sheath called the rectus sheath. This sheath is formed by layers of the aponeurosis of the external and internal oblique and transverse abdominis muscles.

Phew! We've made it to the final muscle of this group – the pyramidalis. Located in the anterior abdominal wall, this pair of triangular muscles are encased within the rectus sheath and reside anterior to the rectus abdominis. Each pyramidalis muscle originates from the pubic crest and the pubic symphysis. These muscles taper as they ascend forming their distinct pyramid shape and insert onto the linea alba. The small pyramidalis muscle receives its innervation from the subcostal nerve. This muscle functions to tense the linea alba. Interestingly, the pyramidalis is quite variable as it is absent altogether in 20 percent of individuals.

Lastly, before we move on, you can commit all of these anterolateral muscles to memory with the mnemonic TIRE Pump. This stands for Transversus abdominis, Internal oblique, Rectus abdominis, External oblique, and Pyramidalis, and this order represents the arrangement of these muscles from deep to superficial.

Okay, now that we have sculpted our knowledge of the anterolateral muscle group, it's time to look at the muscles of the posterior abdominal wall.

The muscles that make up the posterior abdominal wall are the quadratus lumborum, the psoas major, the psoas minor, and iliacus. Now it's important to note that the quadratus lumborum is technically the only true posterior abdominal muscle. Though topographically part of the posterior abdominal wall, the psoas and iliacus muscles actually extend into the lower limb and thus have some different functional classifications but will hone in on their role in just a second.

Though quadratus lumborum sounds reminiscent of a spell from Harry Potter, it's actually a posterior abdominal wall muscle. Originating from the iliac crest and iliolumbar ligament, the quadratus lumborum ascends and inserts onto the inferior border of the last rib and the transverse processes of L1 to L4. This muscle is situated posterior to the psoas muscles while superior to the iliacus and receives innervation from the subcostal nerve and the anterior rami of spinal nerves L1 to L4. The quadratus lumborum muscle aids in depressing and stabilizing the 12th rib while also facilitating extension and lateral bending of the trunk.

Next we have the psoas major muscle. This triangular-shaped and bilaterally paired muscle originates from the vertebral bodies of T12 to L4 and their intervertebral discs as well as the transverse processes of L1 to L5. Extending down to its insertion at the lesser trochanter of the femur, the psoas major is innervated by the anterior rami of spinal nerves L1 to L3.

Situated posterior to the psoas major is the iliacus muscle. It arises from the iliac fossa and spans down to its point of insertion on the lesser trochanter of the femur. You may notice that the iliacus shares an insertion site with the psoas major and together these muscles constitute the iliopsoas muscle which is the most powerful hip flexor in the body, though, the iliacus itself is supplied by the femoral nerve.

At last we've reached the final muscle of the posterior abdominal wall, the minor to our major; that is, the psoas minor. Or have we? Where'd the psoas minor go? I thought it was right there on the screen. Oh, that's right. The psoas minor is variable and often absent. In fact, it's present in only approximately 40 percent of individuals. Anyways, now that we've found an existing psoas minor, you can see it arises from the vertebral bodies of T12 to L1 then it descends and inserts onto the pubic bone, specifically, the iliopubic eminence and pectineal line of the pubis. Lastly, the psoas minor is supplied by the anterior rami of spinal nerves L1 just as its major counterpart. The psoas minor muscle assists in the weak flexion of the lumbar vertebrae.

All right, since we've learned all about the muscles of the abdominal wall, let's explore a clinical note.

You may have heard of a hernia before as they are a fairly common problem. A hernia is the abnormal exit of a tissue or organ through the wall of the cavity in which it usually resides. There are several types of abdominal hernias. One of these types, umbilical hernias, occur when the abdominal contents protrude through the anterior abdominal wall around the umbilicus. Clinically, umbilical hernias present as a bulge near the umbilicus that may or may not be easily detectable.

Congenital or infantile umbilical hernias are rather common and occur because of incomplete closure of the umbilical fibromuscular ring after separation from the umbilical cord. These hernias often resolve with time and the ring is usually obliterated by age two. In contrast, acquired umbilical hernias in adults are typically caused by weakening in the linea alba and can occur following abdominal surgery. Ultrasound or CT imaging can be utilized to detect non-obvious umbilical hernias and treatment for significant herniation include surgical hernia repair to prevent strangulation and necrosis of the protruding abdominal viscera.

Done and dusted! Take a deep breath and engage those abdominal wall muscles you've mastered. But before you take off, let's run through a brisk summary of what we've learnt today.

We kicked things off by looking at the anterolateral abdominal muscles separating this group into the three flat and two vertical abdominal wall muscles. Starting with the three flat wall muscles, we reviewed the external oblique noting its inferomedial muscle fibers running from the lower ribs to its insertions on the iliac crest, pubic bone and linea alba. The external oblique muscle contributes to the formation of the inguinal ligament. Then we discussed the internal oblique and its superomedial muscle fibers extending from the thoracolumbar fascia, iliac crest and inguinal ligament to the lower ribs and the linea alba. We also explored two structures which are formed by contributions of the internal oblique muscle – the conjoint tendon and cremaster muscle.

Next we wrapped up the flat wall muscles with the transversus abdominis. We covered how this is the deepest abdominal muscle spanning horizontally from the lower costal cartilages, thoracolumbar fascia and iliac crest to the linea alba. Traveling through the inferior portion of the anterior abdominal muscles is a short oblique passageway known as the inguinal canal. The deep inguinal ring is formed by an opening in the transversalis fascia of the abdominal wall while the superficial inguinal ring marks the terminal point of the inguinal canal and is formed by a hiatus in the aponeurosis of the external oblique muscle.

Moving on we went over the two vertical wall muscles beginning with the rectus abdominis. Here we discussed its dynamic structure and attachment from the pubic bone extending to both the xiphoid process of the sternum and costal cartilages of ribs 5 to 7. We capped off this group with the most superficial muscle, the small and paired pyramidalis, and its attachments from the pubic bone to the linea alba.

Subsequently, we transitioned to the muscles of the posterior abdominal wall. To start, we looked at the quadratus lumborum, specifically, its posterior location and attachments extending from the iliac crest to the last rib and transverse processes of the upper lumbar vertebrae. We then took a peek at the psoas major muscle spanning from the lumbar vertebrae to the femur. We also reviewed the psoas major's relationship to the iliacus muscle that resides posteriorly within the iliac fossa, specifically, how these muscles are collectively known as the iliopsoas. Finally, we discussed the psoas minor including how it's often absent, only being present in 40 percent of individuals. This muscle runs from the vertebral bodies of T12 to L1 to the iliopubic eminence and the pectineal line of the pubis.

We concluded this tutorial by getting clinical, namely, we reviewed umbilical hernias – their cause, manifestation, and treatment.

Cheers! We've completed the tutorial. I hope you enjoyed it and happy studying!