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The trunk is made up of muscles that move the vertebral column, form the thoracic and abdominal walls, and cover the pelvic outlet. The primary muscles involved in lateral flexion of the torso are the internal and external obliques, the quadratus lumborum, and the erector spinae. Other muscles that assist with the movement include the rectus abdominis, iliopsoas, and semispinalis. When these muscles contract on one side of the body, the torso bends to that side. The facet joints in the thoracic and lumbar spine enable the torso to flex laterally.
| Characteristics | Values |
|---|---|
| Primary muscles involved in lateral flexion of the torso | Internal and external obliques, quadratus lumborum, and erector spinae |
| Other muscles that assist with the movement | Rectus abdominis, iliopsoas, semispinalis |
| Muscles involved in lateral flexion of the lumbar spine | Unilateral action of external oblique, semispinalis, and deep posterior spinal muscles |
| Muscles involved in trunk rotation to the opposite side | Unilateral action of external oblique, semispinalis, and deep posterior spinal muscles |
| Muscles involved in trunk rotation to the same side | Internal oblique and erector spinae |
| Muscles involved in extending the spine bilaterally | Multifidus, erector spinae |
| Muscles involved in unilateral lateral flexion of the spine | Multifidus |
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What You'll Learn
Internal and external obliques
The internal and external obliques are muscles that are part of the abdominal wall. The internal obliques are found deep in the abdominal wall and originate from the iliac crest, the lateral two-thirds of the inguinal ligament, and a sheet of connective tissue in the lower part of the back known as the thoracolumbar fascia. They insert into the lower three or four ribs and the linea alba via aponeurosis. The linea alba is a fibrous band of connective tissue that runs from the xiphoid process to the pubic symphysis.
The external obliques, on the other hand, are located at the side of the trunk and are one of the outermost abdominal muscles. They originate from the lower eight ribs and insert into the anterior half of the iliac crest, the abdominal aponeurosis, the linea alba, and the pubic bone.
Both the internal and external obliques function to flex the trunk and compress its contents. They also work together to laterally flex the trunk and rotate it to the opposite side. For example, when twisting to the right, the left side of the external oblique contracts with the right side of the internal oblique.
The obliques are important for stabilization, especially in the context of abdominal strength and spinal stability. Training the obliques through exercises such as side stretches can help improve flexibility, range of motion, and stability, reducing the risk of injury.
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Quadratus lumborum
The shape of the Quadratus Lumborum is irregular but is generally quadriangular, which is why it is described as 'quadratus' in Latin. It is flattened and has a quadrangular shape. The muscle is an integral part of the thoracolumbar fascia, a myofascial system that covers the posterior area of the human body, involving parts of the lower and upper limbs.
The muscle is innervated by the subcostal nerve (T12) and anterior rami of spinal nerves L1-L4. The blood supply to the Quadratus Lumborum comes from the branches of lumbar, subcostal, median sacral, and iliolumbar arteries. The muscle is often mistakenly referred to as one of the muscles of the back.
The forces exerted by the erector spinae and multifidus in the lumbar vertebrae are about 100 N and 150 N, respectively, while the Quadratus Lumborum exercises only 10 N. Thus, this muscle does not appear to be a significant lumbar stabilizer. As an accessory inspiratory muscle, the Quadratus Lumborum cannot transmit contractile forces to the diaphragm due to the mobility of the last rib.
The Quadratus Lumborum is involved in extending the trunk and flexing it laterally to the ipsilateral side. When the muscle is activated on only one side, the trunk bends towards that direction (lateral flexion). The muscle is one of the primary muscles involved in lateral flexion of the torso.
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Erector spinae
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 plays a crucial role in spinal stability. The ES is formed of three muscles: the iliocostalis, longissimus, and spinalis. These muscles have fibres running vertically through the lumbar, thoracic, and cervical regions. The iliocostalis is the most lateral part of the ES, attaching to the ribs. The longissimus is the central and thickest muscle of the ES, attaching to the transverse process of the vertebrae. The spinalis is the smallest and most medial column, originating from the spinous process of the adjacent vertebrae.
The ES is involved in several important functions. Firstly, it helps to move the vertebral column. Bilateral contraction of the ES extends the spine, while unilateral contraction causes lateral flexion. This means that when the muscles contract on one side of the body, the torso bends to that side. The ES also helps to maintain posture by stabilising the lumbar spine and steadying the spine on the pelvis during walking.
In individuals with low back pain (LBP), the ES can exhibit increased activity to compensate for the instability in the lumbar spine. This increased activity can lead to a higher risk of injury. Additionally, in LBP patients, the ES flexion-relaxation phenomenon is absent, indicating a disruption in the normal functioning of the ES.
The ES can be strengthened through various exercises, including lumbar stabilization exercises, which can help restore the flexion-relaxation phenomenon and reduce low back pain. However, care must be taken to avoid injury to the lower back when targeting the ES directly.
Overall, the Erector Spinae muscle group plays a vital role in spinal stability, movement, and posture, and its dysfunction can contribute to low back pain and an increased risk of injury.
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Rectus abdominis
The rectus abdominis is a long, flat muscle that extends along the entire length of the abdomen, from the rib cage to the pubic bone. It is separated from its fellow on the opposite side by the linea alba, a connective tissue. The rectus abdominis is also informally known as the "abs muscle" and is visible as a "six-pack" in individuals with low body fat. It is paired with the pyramidalis muscle to form the anterolateral abdominal wall, along with the three lateral abdominal muscles: external oblique, internal oblique, and transversus abdominis.
The rectus abdominis is involved in movements of the trunk, such as flexion and rotation, and assists in changing intra-abdominal pressure, which is essential for basic bodily functions like micturition and defecation. It also plays a role in core stability, acting like a natural weight belt to protect the lower back from injury. The muscle is responsible for flexing the lumbar spine, as in a crunch, and assists with breathing and forceful exhalation, as seen after exercise.
The rectus abdominis is innervated by the thoraco-abdominal nerves, which are continuations of the T7-T11 intercostal nerves. These nerves pierce the anterior layer of the rectus sheath, and the sensory supply is from the 7-12 thoracic nerves. The muscle's blood supply comes predominantly from the inferior and superior epigastric arteries, with additional contributions from other vessels.
In terms of its origin and insertion, the rectus abdominis is attached inferiorly by two tendons. The larger tendon attaches to the pubic crest, while the smaller, medial tendon attaches to the pubic symphysis. The muscle fibres then extend vertically superiorly and insert into the xiphoid process of the sternum and costal cartilages of the 5th to 7th ribs. The most lateral fibres typically attach to the anterior end of the 5th rib, but they can extend to the 3rd or 4th ribs in some cases.
To isolate the rectus abdominis during exercise, it is recommended to bend the knees to minimise the engagement of hip flexors. Raising the trunk should be performed in a straight manner.
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Iliopsoas
The iliopsoas muscle is a complex muscle system that can function as a unit or as separate muscles. It is made up of three muscles: the iliacus, psoas major, and psoas minor. The psoas minor is only present in 60-65% of individuals.
The iliopsoas is an anterior or inner hip muscle that is the prime mover of hip flexion, and the strongest of the hip flexors. It is also involved in external rotation of the hip joint, playing an important role in maintaining the strength and integrity of the hip joint. The iliopsoas is essential for correct standing or sitting lumbar posture and stabilising the coxofemoral joint, and is crucial during walking and running.
The iliopsoas muscle is covered by the iliac fascia, which begins as a tube-shaped psoas fascia that surrounds the psoas major muscle as it passes under the inguinal ligament. The psoas major and iliacus muscles join together, passing under the inguinal ligament and inserting onto the femur at the lesser trochanter. The iliopsoas gets innervation from the L2-4 nerve roots of the lumbar plexus, which also send branches to the superficial lumbar muscles. The femoral nerve passes through the muscle and innervates the quadriceps, pectineus, and sartorius muscles.
The iliopsoas tendon can develop tendinopathy, which can cause chronic groin pain and limited hip range of motion. Iliopsoas bursitis involves the expansion of the bursa due to synovial fluid and/or hypertrophic synovium. It is often associated with internal coxa saltans or hip joint disease.
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Frequently asked questions
The internal and external obliques, the quadratus lumborum, and the erector spinae are the primary muscles involved in lateral flexion of the torso.
The internal obliques are muscles that work bilaterally to create flexion of the trunk, compression of the abdominal viscera, expiration, and an increase in intra-abdominal pressure. They can also work unilaterally to create ipsilateral trunk lateral flexion and ipsilateral trunk rotation.
The external obliques are muscles that work bilaterally to create flexion of the trunk, compression of the abdominal viscera, expiration, and an increase in abdominal pressure. They can also work unilaterally to create ipsilateral trunk lateral flexion and contralateral trunk rotation.
The quadratus lumborum is a muscle of the posterior wall, often described as part of the ventral trunk musculature. It is involved in extending the trunk and flexing it laterally to the ipsilateral side.
The erector spinae is a group of muscles on each side of the vertebral column that extends from the sacrum to the skull. They are primarily responsible for extending the vertebral column to maintain an erect posture.
