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The muscles of the trunk assist essential bodily functions such as breathing, movement, defecation, and micturition. The trunk muscles can be categorised into two groups: the superficial or extrinsic muscles of the back, and the deep or intrinsic muscles of the back. The superficial muscles are further divided into two layers: the trapezius, latissimus dorsi, rhomboid major and minor, and levator scapulae muscles; and the serratus posterior superior and inferior muscles. The deep trunk muscles are responsible for maintaining posture and rotating the neck. These muscles include the semispinalis and multifidus muscles.
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What You'll Learn
- The deepest flat muscle, transversus abdominis, assists expiration and causes trunk rotation
- The diaphragm, a dome-shaped muscle, is the main muscle of breathing
- The deep back muscles occupy the space between the spinous and transverse processes of adjacent vertebrae
- The erector spinae group of muscles is responsible for extending the vertebral column to maintain erect posture
- The semispinalis and multifidus muscles are key in maintaining posture
The deepest flat muscle, transversus abdominis, assists expiration and causes trunk rotation
The trunk muscles are responsible for essential body activities such as breathing, movement, defecation, and micturition. The deepest flat muscle, transversus abdominis, assists expiration and causes trunk rotation.
The transversus abdominis is the final muscle in the abdominal area of the anterior trunk. It is the deepest layer of the three layers of muscle that sit over the abdomen. The external oblique muscle is the outer layer, with the internal oblique muscles on the sides of the abdomen and the rectus abdominis muscle in between. The transversus abdominis lies underneath the internal oblique muscle. Its fibres run laterally and are concentrated at the sides of the abdomen.
The transversus abdominis has an extensive origin, with attachments to the costal cartilages of ribs 7-12, thoracolumbar fascia, anterior two-thirds of the iliac crest, and the iliopectineal arch. It crosses the abdomen horizontally to insert onto the linea alba, aponeurosis of the internal abdominal oblique muscle, pubic crest, and pectineal line of the pubis.
Contraction of the transversus abdominis compresses the abdominal organs and assists expiration. It is innervated by the intercostal, subcostal, iliohypogastric, and ilioinguinal nerves. Studies have shown that the transversus abdominis is consistently active during trunk rotation, with varying activity in different muscle regions. For example, during left rotation, there is greater activity in the lower and middle regions of the contralateral transversus abdominis and the lower region of the contralateral obliquus internus. In contrast, during right rotation, the upper region of the ipsilateral transversus abdominis and obliquus externus are predominantly active.
Weakness in the transversus abdominis can lead to symptoms such as an inability to hold in the stomach after a large meal or low back fatigue after prolonged standing or walking. Techniques such as the abdominal draw-in maneuver can help to activate and strengthen this muscle.
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The diaphragm, a dome-shaped muscle, is the main muscle of breathing
The diaphragm is a dome-shaped muscle that separates the thorax and abdomen. It is located below the lungs and is attached to the sternum, the bottom of the rib cage, and the spine. This muscle is the main muscle of breathing, facilitating inhalation and exhalation.
Upon inhalation, the diaphragm contracts and flattens, enlarging the chest cavity and creating a vacuum that pulls air into the lungs. During exhalation, the diaphragm relaxes and returns to its dome shape, forcing air out of the lungs. This process occurs rhythmically and continually, and most of the time, involuntarily.
The diaphragm is made up of three parts: the sternal, costal, and lumbar parts. The sternal part originates from the posterior aspect of the xiphoid process, while the costal part originates from the lower internal surfaces of the costal cartilages and ribs 7-12. The lumbar part, meanwhile, originates from the bodies of vertebrae L1-L3, intervertebral discs, and several ligaments. All these fibres converge on a central tendon in the middle of the trunk, which has no bony insertions.
The diaphragm is critical to breathing and plays a vital role in keeping us healthy. It also helps with other important functions, such as urination and defecation, and preventing acid reflux by putting pressure on the oesophagus.
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The deep back muscles occupy the space between the spinous and transverse processes of adjacent vertebrae
The trunk muscles are essential for several body activities, including breathing, movement, defecation, and micturition. They are divided into two groups: the anterior trunk muscles and the posterior or dorsal trunk muscles. The posterior trunk muscles are commonly referred to as the muscles of the back and are further classified into two functional groups: superficial or extrinsic muscles, and deep or intrinsic muscles.
The diaphragm, a dome-shaped muscle, is also considered a deep back muscle. It forms a partition between the thorax and the abdomen and is the primary muscle responsible for breathing. The intercostal muscles, located in the spaces between the ribs, also play a key role in respiration. The external intercostals elevate the ribs during the inspiration phase, while the internal intercostals can elevate or depress the ribs.
Deep trunk muscle exercises have been shown to have significant effects on foot pressure in healthy adults, particularly in the forefoot and midfoot regions. These exercises can help improve trunk stability, balance, and gait abilities, thereby reducing the risk of physical injury.
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The erector spinae group of muscles is responsible for extending the vertebral column to maintain erect posture
The trunk muscles are essential for several bodily functions, including breathing, movement, defecation, and micturition. 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 erector spinae group of muscles is a large muscle mass that extends from the sacrum to the skull on each side of the vertebral column. These muscles are primarily responsible for extending the vertebral column to maintain an erect posture. The erector spinae is a set of muscles that straighten and rotate the back, working together with the glutes (gluteus maximus, gluteus medius, and gluteus minimus) to maintain a stable posture when standing or sitting.
The erector spinae is not a single muscle but a group of muscles and tendons that run along the spine's length on the left and right sides. They are also known as the sacrospinalis group of muscles. These muscles lie on either side of the spinous processes of the vertebrae and extend throughout the lumbar, thoracic, and cervical regions. The erector spinae is covered in the lumbar and thoracic regions by the thoracolumbar fascia and by the nuchal ligament in the cervical region.
The erector spinae is formed of three muscles: the spinalis, longissimus, and iliocostalis. The longissimus is the central and largest muscle of the erector spinae, attaching to the transverse processes of the vertebrae. The spinalis muscles are the smallest and most medial column, divided into three regional groups: spinalis capitis, spinalis colli, and spinalis thoracis. The iliocostalis muscles are the most lateral part of the erector spinae, attaching to the ribs.
The erector spinae muscles are essential for spinal stability and posture control. Bilateral contraction of these muscles extends the spine, while unilateral contraction causes lateral flexion. They help maintain posture by steadying the spine on the pelvis during walking. However, these muscles are also commonly affected by back strains due to incorrect balancing of loads on the vertebral column. Adequate warm-up, stretching, and exercises are crucial for preventing back strains and pain by stabilizing the vertebral column.
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The semispinalis and multifidus muscles are key in maintaining posture
The trunk muscles assist essential body activities such as breathing, movement, defecation, and micturition. 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 posterior trunk muscles are more commonly referred to as the muscles of the back. They are divided into two functional groups: superficial or extrinsic muscles of the back, and deep or intrinsic muscles of the back.
The semispinalis and multifidus muscles play an important role in spinal stability. The multifidus spans the whole length of the vertebral column but is most developed in the lumbar area. The deepest layer of the multifidus appears to contribute more strength and stability to the spine than the superficial layers. The multifidus functions as a stabilizer during trunk rotation, opposing trunk flexion and maintaining a pure axial rotation. Bilateral contraction of the multifidus produces extension of the vertebral column at all levels, while unilateral contraction aids in lateral flexion and rotation of the spine to the opposite side. The semispinalis thoracis, cervicis, and capitis extend the neck and head, laterally flex the neck and head, and contralaterally rotate the neck at the spinal joints.
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Frequently asked questions
Deep trunk muscles are the muscles that act on the trunk. They are divided into two groups: superficial or extrinsic muscles of the back, and deep or intrinsic muscles of the back. The deep trunk muscles include the semispinalis, multifidus, and the diaphragm.
The deep trunk muscles are responsible for maintaining posture and rotating the neck. They also assist with essential body activities such as breathing, movement, defecation, and micturition.
Deep trunk muscle exercises can be performed under the supervision of a physical therapist. One example of a deep trunk muscle exercise is lying down, lifting both legs 70-80 degrees off the ground, and lifting the upper body while waving both arms 5 times with inhaling and exhaling.
