Definition
Oblique muscle refers to two abdominal muscles – the external and internal obliques. These provide trunk flexion and rotation. The external oblique is the thickest and runs from the lower ribs to the iliac crest. The internal oblique lies under the external muscle and also originates at the iliac crest before reaching the pubic bone. Here, it joins a sheet of connective tissue shared with the transversus abdominis muscle.
Where is the Oblique Muscle Located?
The oblique muscle group is located in the anterolateral abdominal wall. We also have a paired inferior oblique muscle and superior oblique muscle that are part of the extrinsic muscles of the eye orbit. This article only describes the obliques of the lateral abdominal wall.
Oblique muscle range of motion also implements an underlying muscle called the transversus abdominis muscle, the deepest and thinnest of this group of three trunk flexion and rotation muscles.
The anterolateral abdominal wall is composed of various anatomical structures:
- External abdominal obliques
- Internal abdominal obliques
- Transversus abdominis musclues
- Anterior abdominal muscles
- Rectus abdominis muscles
- Pyramidalis muscles
- Transversalis fascia
- Peritoneum
- Camper’s fascia (fatty tissue)
- Scarpa’s fascia (membranous tissue)
- Skin
A fascia is a sheet of connective tissue that helps muscles to attach to other structures, withstands high tension forces, and also covers muscles to increase their stability. In the anterolateral abdominal wall, these layers allow a gliding motion that reduces friction; this increase oblique and transversus abdominis muscle efficiency.
The peritoneum lines the abdominal cavity, covering the internal organs.
Oblique Muscle Group
When talking about the oblique muscle group, it is also worth mentioning the transversus abdominis. This is not only because it contributes to oblique muscle range of motion but also because it is indirectly connected to the internal oblique by way of a shared aponeurosis.
An aponeurosis is another sheet-like, strong connective tissue that acts a little like a fascia. It connects muscle to bone or muscle to fascia.
The abdominal obliques are important in maintaining abdominal wall tension and support the internal organs. If these muscles are weakened or damaged, the result could be an abdominal hernia where soft tissues underneath the muscles bulge out.
Abdominal wall tension is important when breathing out forcefully, urinating, defecating, and – in females – during childbirth.
External Oblique Muscle
The external oblique muscle is a paired muscle that runs outside ribs five to twelve down to the crest of the ilium bone. When the left side contracts, the right relaxes. This allows for ipsilateral flexion (same side) to the contracting side and contralateral rotation (opposite side) of the trunk.
When this muscle contracts on both sides (bilateral contraction) it works together with the rectus abdominis and internal oblique muscles to flex the trunk forward. This movement also contributes to breathing. You can test this by standing or sitting and forcefully breathing out. You will feel your trunk move forward. This is due to the direction of the muscle fibers that extend downward that the lower ribs are pulled toward the pelvis.
External oblique muscle origin is at the external surfaces of the fifth to twelfth ribs. Insertion points are via the external oblique aponeurosis at the linea alba, the front of the iliac crest, and the pubic tubercle (see image below). The linea alba or white line is a fibrous connective tissue that runs through the middle of the abdomen.
Internal Oblique Muscle
The paired internal oblique muscle sits underneath the external oblique. It originates at the front of the iliac crest, the lumbodorsal fascia, and the iliopectineal fascia between the groin and hip bone.
From these origins, the internal oblique muscle inserts at ribs ten, eleven, and twelve, at the linea alba, the pubic crest, and the pectineal line.
Unlike the external obliques, the internal muscle has fibers that go in different directions:
- Anterior fibers from the iliopectineal fascia to the pubic bone.
- Lateral fibers from the front of the iliac crest to the linea alba.
- Posterior fibers from the back of the iliac crest and lumbodorsal fascia to the lower ribs.
When one side of the internal oblique contracts, it assists with flexion and rotation to that side. When both contract, they assist in trunk flexion and forced expiration.
Transversus Abdominis
Although not an oblique muscle, the transversus abdominis plays an important supporting role. It is another paired sheet of muscle that lies under the internal oblique; the transversus abdominis has horizontal fibers.
This muscle inserts from the seventh to the twelfth rib, the lumbodorsal fascia, the front of the iliac crest, and the iliopectineal fascia.
It inserts at the linea alba, the aponeurosis of the internal oblique, the pubic crest, and the pectineal line. These are origins and insertions that match those of both the internal and external abdominal obliques.
The transversus abdominis contracts at both sides to support the internal organs and provide additional expiration forces. When contracted on one side it assists with trunk rotation to the ipsilateral (same) side.
Oblique Muscle Strain
Oblique muscle strain most commonly occurs on the opposite side to the direction of motion. For example, if turning the trunk quickly to the right, the left obliques are most likely to be strained.
Abdominal oblique injuries or side strains are associated with sports that involve trunk rotation. A particularly enthusiastic swing of the golf club, tennis racket, or baseball bat can cause a strain injury in the anterolateral abdominal wall muscles.
Symptoms are a sudden, sharp pain on the contralateral side close to the lower ribs. Oblique muscle pain treatment includes alternating heat and cold therapy, pain-relieving medication, gentle exercise, and compression.
Oblique muscle spasm causes significant pain and breathlessness. Spasms are usually associated with sports injuries but may also be caused by nerve damage.
Oblique Muscle Exercises
To carry out oblique muscle exercises correctly, you need to understand how these muscles work. As we have learned, the principle oblique range of motion is flexion and rotation.
The best oblique muscle workout is the bicycle crunch. In this exercise, the person lies on the ground, places both hands behind the head, bends the knees, and brings one elbow toward the opposite knee as seen below.
Those who have done this exercise without warming up the muscles first may find themselves having to deal with an oblique injury. When performed correctly, this exercise reduces the risk of acquired abdominal hernias, as does losing belly fat.
Obesity stretches the abdominal fascia and weakens it; yo-yo dieting is particularly associated with ventral hernias. Doctors can test for hidden ventral hernias by asking their patient to perform the Valsalva maneuver which increases intra-abdominal pressure and forces internal soft tissues through any oblique muscle weak spots.
