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The muscles of the trunk are divided into two groups: superficial (extrinsic) muscles that move the upper limbs, and deep (intrinsic) muscles that act on the trunk. The primary muscles involved in flexing the trunk are the internal and external obliques, the quadratus lumborum, and the erector spinae. Other muscles that assist with trunk flexion include the rectus abdominis, iliopsoas, and semispinalis. The abdominal muscles are the prime movers of trunk flexion, and optimal abdominal muscle function is necessary for several life functions, including breathing and lifting.
| Characteristics | Values |
|---|---|
| Muscles of the trunk | Rectus Abdominis, External Oblique, Internal Oblique, Transversus Abdominis, Quadratus Lumborum, Erector Spinae, Intercostal Muscles, Diaphragm |
| Function | Flexion, Rotation, Compression of Abdominal Viscera, Expiration, Lateral Flexion, Extension |
| Innervation | Intercostal, Subcostal, Iliohypogastric, Ilioinguinal |
| Structural Groups | Flexors, Extensors, Right/Left Lateral Flexors, Right/Left Rotators |
| Location | Anterior, Posterior |
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What You'll Learn
The abdominal muscles are the prime movers of trunk flexion
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. It also compresses the abdominal contents to protect them. The external and internal oblique muscles are also involved in trunk flexion. They originate from the external and internal surfaces of ribs 5-12, respectively, and insert onto the anterior half of the iliac crest and pubic tubercle.
The abdominal oblique muscles are involved in ipsilateral rotation and compression of abdominal viscera. They assist with expiration and are innervated by the intercostal, subcostal, and iliohypogastric nerves. The final muscle in the abdominal area of the anterior trunk is the transversus abdominis muscle. While the abdominal muscles are the prime movers of trunk flexion, they also perform multiple other key functions. For example, the abdominal muscles can assist in forced expiration by contracting and elevating the ribs.
The muscles of the back are divided into two groups: superficial (extrinsic) muscles that move the upper limb, and deep (intrinsic) muscles that act on the trunk. The superficial layer contains the trapezius, latissimus dorsi, rhomboid major and minor, and levator scapulae muscles. The intermediate layer possesses serratus posterior superior and inferior muscles. The latissimus dorsi muscle is also involved in trunk flexion, along with other muscles such as the erector spinae, semispinalis thoracis, and deep posterior spinal muscles.
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The internal and external obliques are involved in trunk flexion
The trunk is moved by two groups of muscles: superficial (extrinsic) muscles and deep (intrinsic) muscles. The superficial muscles move the upper limb, while the deep muscles act on the trunk. The muscles of the thoracic wall, for example, are involved in the breathing process.
The external abdominal oblique has a variety of functions depending on whether it contracts unilaterally or bilaterally. When contracting unilaterally, it rotates the trunk to the opposite side. When contracting bilaterally, it works together with the internal abdominal oblique and rectus abdominis to flex the trunk anteriorly. This bilateral contraction also increases intra-abdominal pressure, which is useful for processes such as breathing, singing, and defecation.
The internal abdominal oblique also has several functions depending on which parts of the muscle contract. When contracting bilaterally, it flexes the trunk and compresses the intra-abdominal viscera, increasing intra-abdominal pressure. This action is used for functions such as forced expiration, micturition, and defecation. When contracting unilaterally, it causes ipsilateral flexion and rotation of the trunk.
In addition to their role in trunk flexion, the internal and external obliques are also involved in ipsilateral rotation and compression of abdominal viscera. They assist with expiration and contribute to changing intra-abdominal pressure, which is essential for basic bodily functions.
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The rectus abdominis is a flexor of the trunk
The rectus abdominis is a long, strap-like muscle that passes vertically over the entire length of the abdomen. It is a flexor of the trunk, and it acts to flex the spinal column. It is involved in movements of the trunk, mainly flexion and rotation. It also contributes to changing the intra-abdominal pressure, which is essential for basic bodily functions such as forced breathing, defecation, and micturition.
The rectus abdominis is one of the five main abdominal muscles, which help hold organs in place and support the body during movement. These muscles make up the core muscles, along with the back muscles, and they help protect the spine and keep the body stable and balanced. The abdominal muscles are strong bands of muscles lining the walls of the abdomen, located towards the front of the body between the ribs and pelvis.
The rectus abdominis is the top layer of the abdominal muscles, often referred to as the
To isolate the rectus abdominis during exercise, one can perform a sit-up exercise while bending the knees to minimize the engagement of hip flexors. This involves raising the trunk against gravity to the midline and slowly lowering it down repeatedly in multiple sets.
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The erector spinae extends, laterally flexes, and rotates the trunk
The erector spinae is a group of muscles that extend, laterally flex, and ipsilaterally rotate the trunk at the spinal joints. It is composed of three subgroups: iliocostalis, longissimus, and spinalis. These muscles are deep muscles of the back that extend and laterally flex the spine. They are the most superficial of the posterior trunk muscles, ascending vertically on either side of the vertebral column.
The spinalis muscles are the most medial erector spinae muscles. They 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. They are innervated by the lateral branches of the posterior rami of the cervical, thoracic, and lumbar spinal nerves.
The longissimus muscles are the central erector spinae muscles and are the thickest and longest. They are also divided into three regional groups, from superior to inferior: longissimus capitis, longissimus colli, and longissimus thoracis. The longissimus muscle forms the main meat of the erector group and attaches along the transverse process of the vertebrae.
The iliocostalis muscles are the most lateral part of the erector spinae muscles and attach to the ribs. They function to extend and laterally flex the spine. Similar to the other erector spinae groups, they are innervated by the lateral branches of the posterior rami of the cervical, thoracic, and lumbar spinal nerves.
The erector spinae muscles play an important role in spinal stability and low back pain. In healthy individuals, the erector spinae muscles relax in a range from an upright position to full lumbar flexion. However, in individuals with low back pain, the erector spinae flexion-relaxation phenomenon is absent, and there is decreased activity and atrophy of the multifidus muscle, compromising spinal stability.
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The quadratus lumborum flexes the trunk to the side
The quadratus lumborum is a muscle of the posterior abdominal wall, lying deep inside the abdomen and dorsal to the iliopsoas. It is often mistakenly referred to as one of the muscles of the back. The muscle 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 L1-L4 vertebrae. The quadratus lumborum is involved in extending the trunk and flexing it to the side (lateral flexion). This action is possible due to inputs provided by the subcostal nerve and the anterior rami of the L1-L4 spinal nerves.
The quadratus lumborum 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 muscle is flat and quadrangular in shape, with three muscle fibre layers that have different vectors. The thin anterior layer comprises iliocostal and iliothoracic muscle fibres, which run from the iliac crest to the ribs and the T12 vertebral body's lateral margin, respectively. The middle layer is made up of lumbocostal muscle fibres, which originate from the transverse processes of the lumbar vertebrae and insert on the 12th rib.
The quadratus lumborum is involved in the stabilisation and movement of the spine and pelvis. When the muscle contracts bilaterally, it extends the lumbar vertebral column. When it is activated on one side only, the trunk flexes towards that side. In addition, the quadratus lumborum fixes the 12th rib during movements of the thoracic cage, supporting expiration.
The muscle is also important in clinical settings, particularly for anaesthesia during surgery. Its strategic position and the entropic scheme of its fibres make it a significant means of accessing the back, lower limbs, or abdominal area. However, uncertainty remains about whether abnormalities in the quadratus lumborum are a primary source of back pain.
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Frequently asked questions
The muscles that flex the trunk are the rectus abdominis, external and internal oblique, and psoas major.
Trunk flexion is the most pronounced spinal motion. About 75% of all spinal flexion below the neck occurs in the lumbar spine.
The internal and external obliques, the quadratus lumborum, and the erector spinae are the primary muscles involved in lateral flexion of the torso.
The superficial muscles of the back are the trapezius, latissimus dorsi, rhomboid major and minor, and levator scapulae muscles.
The six major functional groups of the lumbar spine are flexors and extensors in the sagittal plane, right and left lateral flexors in the frontal plane, and right and left rotators in the transverse plane.
