Maximus Gage
The human body is a complex system of muscles, bones, and ligaments that work together to produce movement. One of the key components of the body's structural framework is the trunk, which is made up of the chest, abdomen, and pelvis. The trunk muscles assist in essential body activities such as breathing, movement, defecation, and micturition. While there are many muscles that contribute to the movement of the trunk, one of the primary muscles responsible for extending the trunk is the quadratus lumborum. This muscle originates from the iliac crest and iliolumbar ligament and inserts onto the inferior border of the 12th rib, also attaching to the transverse processes of L1-L4 vertebrae. Another important muscle group for trunk extension is the erector spinae, which helps to extend the spine and maintain an upright posture.
| Characteristics | Values |
|---|---|
| Muscle that extends the trunk | Erector spinae |
| Location | Vertebral column, on each side |
| Function | Extends the spine, returns the body to an upright position |
| Other muscles that extend the trunk | Quadratus lumborum, latissimus dorsi, multifidus, semispinalis, rotatores, rectus abdominis, obliques |
| Other functions of the above muscles | Lateral flexion, rotation, tilting of the pelvis, elevation of the scapula, rib cage movement, respiration |
| Core muscles | Transversus abdominis, rectus abdominis, obliques |
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What You'll Learn
Erector spinae muscles
The Erector Spinae (ES) is a large superficial muscle that lies just deep to the thoracolumbar fascia. It is one of the core and paraspinal muscles and is a common aponeurosis that blends with the thoracolumbar fascia, with a proximal attachment on the sacrum and the spinous processes of the lumbar vertebrae. The ES is formed of three muscles with its fibres running more or less vertically throughout the lumbar, thoracic, and cervical regions.
The Erector Spinae muscles are divided into three groups, from medial to lateral: Spinalis, Longissimus, and Iliocostalis. The Spinalis muscles are the most medial Erector Spinae muscles and are divided into three regional groups, from superior to inferior: Spinalis Capitis, Spinalis Colli, and Spinalis Thoracis. The Spinalis muscles function to extend and laterally flex the cervical and thoracic regions of the spine. The Longissimus muscles are the central Erector Spinae muscles and are also the thickest and longest. They are divided into three regional groups, from superior to inferior: Longissimus Capitis, Longissimus Colli, and Longissimus Thoracis. The Longissimus muscles function to extend and laterally flex the spine and Longissimus Capitis helps to rotate the head ipsilaterally. The Iliocostalis muscles are the most lateral Erector Spinae muscles and are regionally divided into three groups, from superior to inferior: Iliocostalis Colli, Iliocostalis Thoracis, and Iliocostalis Lumborum.
The Erector Spinae muscles play an important role in spinal stability and low back pain (LBP). In healthy individuals without low back pain, the Erector Spinae muscles relax in a range from an upright position to full lumbar flexion, due to the deep back muscles (multifidus) acting to stabilize the lumbar spine. However, in individuals with low back pain, the Erector Spinae flexion-relaxation phenomenon is absent. Lumbar stabilization exercises can help restore the Erector Spinae flexion-relaxation phenomenon by strengthening the multifidus muscle.
The Erector Spinae muscles are involved in trunk extension, along with the semispinalis and deep posterior spinal muscles. Bilateral contraction of the Erector Spinae muscles causes back and head extension, while unilateral contraction causes lateral flexion. They also help to maintain posture by steadying the spine on the pelvis during walking.
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Quadratus lumborum
The Quadratus Lumborum originates from the iliac crest and iliolumbar ligament and inserts onto the inferior border of the 12th rib. As it ascends, it also attaches to the transverse processes of the lumbar vertebrae L1-L4. The muscle is an integral part of the thoracolumbar fascia, a myofascial system that covers the posterior trunk and involves parts of the lower and upper limbs.
The Quadratus Lumborum is involved in extending the trunk and flexing it laterally to the ipsilateral side. It is also described as a lumbar spine extensor, stabiliser, and lateral tiltor, and an accessory muscle for inspiration. However, its role in lumbar extension has been questioned due to the relatively small force it exerts during this movement.
Overuse and strain of the Quadratus Lumborum are major causes of chronic pain in the lower back. Sitting at a desk with a reclined seat is a common cause, as it releases and weakens the intrinsic back muscles, leading to painful tension and stiffening of the Quadratus Lumborum. Understanding the anatomy and clinical information of this muscle is essential for treating various musculoskeletal conditions of the lower back.
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Latissimus dorsi
The latissimus dorsi is a large, flat muscle that stretches across the back and is commonly known as "lats", especially among bodybuilders. It is the widest muscle in the human body. The name "latissimus dorsi" comes from Latin and means "broadest of the back".
The latissimus dorsi is found superficially in the lower two-thirds of the trunk, covering the serratus posterior muscles. It is a superficial layer of the extrinsic back muscles. The muscle is attached to the dorsal vertebrae, with the number of vertebrae varying from four to eight. It is also attached to the costal cartilage, with the number of costal attachments also varying. The latissimus dorsi crosses the inferior angle of the scapula.
The latissimus dorsi is responsible for extension, adduction, transverse extension (also known as horizontal abduction or horizontal extension), flexion from an extended position, and medial internal rotation of the shoulder joint. It also has a role in the extension and lateral flexion of the lumbar spine. The muscle pulls the trunk upward and forward when the arms are in a fixed overhead position.
The latissimus dorsi is supplied by the thoracodorsal artery, a continuation of the subscapular artery, and also receives blood supply from the dorsal perforating branches of the posterior intercostal arteries and the superior lumbar arteries. The thoracodorsal nerve, a branch of the posterior cord of the brachial plexus, provides innervation to the muscle.
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Rectus abdominis
The rectus abdominis is a long, flat muscle that makes up the top layer of the abdominal muscles, commonly referred to as the "six-pack". It is separated from its fellow on the opposite side by the linea alba, a connective tissue. The rectus abdominis is responsible for flexing the spinal column, tensing the anterior wall of the abdomen, and assisting in compressing the contents of the abdomen. It also assists with breathing and plays an important role in respiration during forceful exhalation, such as after exercise.
The rectus abdominis muscle is innervated by the thoraco-abdominal nerves, which are continuations of the T7-T11 intercostal nerves. The sensory supply is from the 7-12 thoracic nerves. The muscle is supplied by a number of vessels, predominantly the inferior and superior epigastric arteries. The rectus abdominis is paired and runs vertically on either side of the linea alba, on the anterior surface of the abdominal wall.
Inferiorly, the rectus abdominis is attached by two tendons. The larger one is attached to the pubic crest, from the pubic tubercle to the pectineal line, while the smaller, medial tendon is attached to the pubic symphysis. The fibres of the rectus abdominis then extend vertically superiorly and insert into the xiphoid process of the sternum and the costal cartilages of the 5th, 6th, and 7th ribs. The most lateral fibres attach to the anterior end of the 5th rib, but in some cases, they may extend to the 3rd and 4th ribs.
The rectus abdominis is similar to the abdominal muscles of most vertebrates. It can be isolated by performing exercises with bent knees to minimise the engagement of the hip flexors. The sit-up exercise, for example, requires raising the trunk against gravity to the midline and slowly lowering it down repeatedly in multiple sets.
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Intercostal muscles
The external intercostal muscles, also known as intercostalis externus, aid in quiet and forced inhalation. They originate on ribs 1–11 and insert on ribs 2–12. The muscle fibres are directed downwards, forwards, and medially in the anterior part. The function of these muscles is to elevate the ribs, expanding the transverse dimensions of the thoracic cavity.
The internal intercostal muscles, or intercostalis internus, aid in forced expiration. They originate on ribs 2–12 and insert on ribs 1–11. Their fibres pass anterior and superior from the upper margin of the rib and costal cartilage to the low rib below. The internal intercostals are responsible for depressing the ribs, bending them inward, and decreasing the transverse dimensions of the thoracic cavity.
The innermost intercostal muscles, also known as intercostalis intimus, are deep layers of the internal intercostal muscles, separated by a neurovascular bundle. Their fibres are directed downwards, forwards, and laterally, similar to the internal intercostals. The innermost intercostals originate from the costal groove of the ribs and insert onto the superior border of the rib below.
Intercostal muscle strains can occur due to weakened muscles, overexertion, direct trauma, or repetitive torso twisting. Symptoms include sudden severe pain, muscle tension and stiffness, and worsening pain within days or weeks. Strains can affect the breathing pattern, causing shallow short breaths and reduced blood oxygenation.
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Frequently asked questions
The erector spinae group of muscles on each side of the vertebral column is responsible for extending the trunk. The latissimus dorsi, rectus abdominis, and transversospinalis group of muscles, which include the multifidus and rotatores, also extend the trunk.
The erector spinae muscles extend the spine and return the body to an upright position from a forward bend.
The muscles of the trunk include those that move the vertebral column, the muscles that form the thoracic and abdominal walls, and those that cover the pelvic outlet. The diaphragm, obliques, and intercostal muscles are also part of the trunk.
