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The muscles of the trunk are divided into two groups: superficial (extrinsic) muscles, which move the upper limb, and deep (intrinsic) muscles, which act on the trunk. The abdominal muscles are the prime movers of trunk flexion, and optimal abdominal muscle function is necessary for several life functions, including lifting and carrying tasks. The primary muscles involved in lateral flexion of the torso are the internal and external obliques, the quadratus lumborum, and the erector spinae. The rectus abdominis, iliopsoas, and semispinalis assist with movement. This group of muscles also contributes to changing the intra-abdominal pressure, which is essential for basic bodily functions such as micturition and defecation.
| Characteristics | Values |
|---|---|
| Muscles involved in trunk flexion | Rectus abdominis, Abdominal oblique muscles (external and internal), Quadratus lumborum, Erector spinae, Iliopsoas, Semispinalis |
| Functions | Flexion, Rotation, Compression of abdominal viscera, Expiration, Changing intra-abdominal pressure |
| Innervation | Intercostal, Subcostal, Iliohypogastric, Ilioinguinal |
| Sex Differences | No significant difference in fatigability and torque steadiness of trunk flexor muscles between males and females |
| Treatment for Tightness | Muscle Energy Techniques (MET), Isometric Contraction Techniques |
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What You'll Learn
Rectus Abdominis
The rectus abdominis muscle, also known as the ""abdominal muscle" or "abs", is a pair of segmented skeletal muscles on the front of a person's abdomen. It is a long, flat muscle that extends along the whole length of the abdomen, from the rib cage to the pubic bone. The rectus abdominis is contained in the rectus sheath, a fibrous compartment that also contains the pyramidalis muscle. The rectus abdominis is separated from the pyramidalis muscle by a band of dense connective tissue called the linea alba, which runs down the middle of the rectus abdominis. The connective tissue defining each lateral margin of the rectus abdominis is the linea semilunaris.
The rectus abdominis is a very important muscle for core stability and acts as a flexor of the trunk. It also assists with breathing and plays a role in respiration when forcefully exhaling, as seen after exercise. The muscle is involved in flexing the spinal column and tensing the anterior wall of the abdomen, as well as assisting in compressing the contents of the abdomen. This compression increases intra-abdominal pressure, which is essential for basic bodily functions such as micturition and defecation.
The rectus abdominis is a paired muscle that runs vertically, with each rectus abdominis traversed by bands of connective tissue called the tendinous intersections, which divide the muscle into distinct muscle bellies. These fibrous bands result in a grid-iron ‘six-pack’ shape in those with low body fat. The muscle is innervated by thoraco-abdominal nerves, which are continuations of the T7-T11 intercostal nerves. The blood supply to the rectus abdominis arises from a number of vessels, but predominantly the inferior and superior epigastric arteries.
The rectus abdominis can be isolated by performing exercises with bent knees to minimise the engagement of hip flexors. This muscle is often assessed for strength rather than flexibility, as weak abdominal muscles can lead to serious problems.
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Abdominal Oblique Muscles
The abdominal oblique muscles are essential for various functions, including trunk flexion, rotation, and maintaining abdominal pressure. These muscles are divided into two groups: the external oblique muscles and the internal oblique muscles.
The external oblique muscles are the largest and outermost of the three flat abdominal muscles of the lateral anterior abdomen. They originate from the external surfaces of ribs 5-12 and insert onto the anterior half of the iliac crest, pubic tubercle, and linea alba. The external obliques are responsible for pulling the chest downwards, which compresses the abdominal cavity and increases intra-abdominal pressure. This pressure change is essential for basic bodily functions such as breathing, defecation, coughing, and vomiting. The external oblique muscles also contribute to trunk flexion and ipsilateral (same-side) rotation, allowing movements like side-bending and rotation to the opposite side.
The internal oblique muscles, on the other hand, are located underneath the external obliques on each side of the trunk. They originate from the anterior two-thirds of the iliac crest, the iliopectineal arch, and the thoracolumbar fascia. These muscles insert onto the inferior borders of ribs 10-12, linea alba, and other structures. Similar to the external obliques, the internal oblique muscles assist in trunk flexion and rotation, but they rotate ipsilaterally.
Both the external and internal oblique muscles work together to enable the trunk to twist and turn. They also play a crucial role in stabilising the trunk and maintaining consistent internal abdominal pressure. This stability and pressure regulation help protect the spine and keep the body balanced during various movements.
Injuries to the abdominal oblique muscles, such as strains or hernias, can be debilitating due to their involvement in a wide range of movements. Oblique strains are common in sports that involve pitching or batting, such as baseball. Maintaining abdominal muscle health is important to prevent injuries and ensure the proper functioning of these muscles in supporting the trunk and regulating internal pressure.
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Quadratus Lumborum
The quadratus lumborum (QL) is a muscle in the posterior inferior trunk lateral to the spine. It is the deepest abdominal muscle and is commonly referred to as a back muscle. The muscle is generally located medial to the aponeurosis of the transverse abdominal muscle and is a continuation of it. The muscle fibres of the quadratus lumborum have a highly variable arrangement, preventing the recognition of a precise orientation pattern.
The quadratus lumborum originates from the iliac crest and iliolumbar ligament. It inserts onto the inferior border of the 12th rib and, as it ascends, it also attaches to the transverse processes of L1-L4 vertebrae. The muscle is flat and quadrangular in shape, and each of the pair is an irregular quadrilateral, hence the name. The number of attachments to the vertebrae and the extent of its attachment to the last rib may vary.
The quadratus lumborum is involved in extending the trunk and flexing it laterally to the ipsilateral side. It is described as an extensor of the lumbar spine, a stabilizer of the lumbar area, capable of pelvic tilting laterally, and an accessory muscle for inspiration. The muscle helps to create an antagonist force to the muscles of the anterolateral abdomen wall, working with the multifidus and erector spinae muscles.
The forces exerted by the quadratus lumborum are relatively small compared to other muscles in the lumbar region, such as the erector spinae and multifidus. This suggests that the quadratus lumborum may not be a significant lumbar stabilizer. However, its strategic position and fibre arrangement indicate that it likely serves as a junction for the forces exerted by neighbouring muscles, influencing the vectors of various tensions.
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Intercostal Muscles
The external intercostal muscles, or intercostalis externus, are the outermost layer of intercostal muscles. They originate from ribs 1–11 and insert onto ribs 2–12. During inhalation, these muscles elevate the ribs and bend them open, expanding the thoracic cavity. The muscle fibres are directed downwards, forwards, and medially in the anterior part. The external intercostals are responsible for quiet and forced inhalation and are facilitated by the anterior and posterior intercostal arteries.
The internal intercostal muscles, or intercostalis internus, are the intermediate layer of intercostal muscles. They originate from the costal groove of one rib and insert onto the superior border of the rib below. These muscles aid in forced expiration by depressing the ribs and bending them inward, thus decreasing the size of the thoracic cavity. The internal intercostals are innervated by the intercostal nerves and receive their blood supply from the anterior and posterior intercostal arteries, as well as other blood vessels.
The innermost intercostal muscles, or intercostalis intimus, are the deepest layer of intercostal muscles. They originate from the costal groove of one rib, posteriorly to the origin of the internal intercostals, and insert onto the superior border of the rib below. The innermost intercostals assist the internal and external intercostals in their functions. They are innervated and vascularised in the same way as the internal intercostals.
Intercostal muscle strains can occur due to weakened muscles, overexertion, direct trauma, or repetitive torso twisting. Symptoms of an intercostal muscle strain include pain in the upper back or rib cage, muscle tension and stiffness, and difficulty breathing due to pain.
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Superficial and Deep Muscles
The muscles of the back are divided into two groups: superficial (extrinsic) muscles and deep (intrinsic) muscles. The superficial muscles move the upper limb, while the deep muscles act on the trunk.
The superficial muscles are further divided into two layers. The superficial layer contains the trapezius, latissimus dorsi, rhomboid major and minor, and levator scapulae muscles. The trapezius is a large, paired, triangular muscle located in the upper back and neck. It consists of three parts that fan out during their course. The descending part originates from the medial third of the superior nuchal line, the external occipital protuberance, nuchal ligament, and cervical vertebrae. It inserts into the lateral third of the clavicle. This part of the trapezius supports and draws the scapula superomedially, extends, rotates, and laterally flexes.
The intermediate layer possesses serratus posterior superior and inferior muscles.
The deep muscles of the back, also known as the intrinsic muscles, act directly on the trunk, allowing for movements such as flexion, extension, and rotation. These muscles include the abdominal muscles, which are essential for several life functions, including breathing and lifting. The abdominal muscles are the prime movers of trunk flexion and include the rectus abdominis, external oblique, internal oblique, and transversus abdominis muscles. The rectus abdominis is a long, strap-like, paired muscle that passes vertically over the entire length of the abdomen. It acts as a flexor of the trunk and an accessory muscle of expiration, helping to compress and protect the abdominal contents. The external and internal oblique muscles originate from the external surfaces of ribs 5-12 and insert onto the anterior half of the iliac crest, pubic tubercle, and linea alba. These muscles are involved in trunk flexion, ipsilateral rotation, compression of abdominal viscera, and expiration.
Other deep muscles that contribute to trunk flexion include the quadratus lumborum, which is part of the posterior wall but is often described as part of the ventral trunk musculature. This muscle is involved in extending the trunk and flexing it laterally to the ipsilateral side.
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Frequently asked questions
The abdominal muscles are the prime movers of trunk flexion. The primary muscles involved in lateral flexion of the torso are the internal and external obliques, the quadratus lumborum, and the erector spinae.
The internal oblique muscles originate from the anterior two-thirds of the iliac crest, the iliopectineal arch, and the thoracolumbar fascia. The external oblique muscles originate from the external surfaces of ribs 5-12.
Optimal function of the abdominal muscles is necessary for several life functions, including lifting and carrying tasks. They also assist with expiration and changing intra-abdominal pressure, which is essential for basic bodily functions such as micturition and defecation.
The rectus abdominis, iliopsoas, and semispinalis muscles assist with trunk flexion. The diaphragm is also a crucial muscle that forms a partition between the thorax and the abdomen.
While sex differences in strength and fatigability have been observed in many limb muscles, it is unclear if such differences exist for the abdominal muscles. Some studies suggest that males and females exhibit similar fatigability and torque steadiness of the trunk flexor muscles during isometric contractions.
