Muscles of the trunk

Muscles of the thoracic cage

The muscles of the thoracic cage are the pectoralis major, pectoralis minor, serratus anterior, subclavius, intercostal (external, internal and innermost), subcostal and transversus thoracis muscles, including the diaphragm.

These muscles attach the upper limb to the axial skeleton of the trunk and support the thoracic cage. They are also involved in movements of the upper limb and in respiration. 

Pectoralis major

Pectoralis major is a large, fan-shaped, superficial muscle located on the anterior thoracic wall. It forms the bulk of the chest area and can be easily seen on the surface in some people, for example weightlifters. The muscle has three heads giving it three points of origin:

• Clavicular head - originates from the anterior surface of the clavicle (medial half).
• Sternocostal head - originates from the anterior surface of the sternum and costal cartilages of ribs 1-6.
• Abdominal (rectus) head - originates from the anterior layer of the rectus sheath.

All the fibers converge and insert on to the crest of greater tubercle of the humerus. Pectoralis major is innervated by the lateral and medial pectoral nerves, two branches of the brachial plexus. The pectoralis major is involved in a variety of arm movements, such as adduction, internal rotation, flexion and extension. It also draws the scapula anteroinferiorly.

Pectoralis minor

Deep to pectoralis major is the pectoralis minor muscle. Pectoralis minor originates from the anterior aspect of ribs 3-5 and their costal cartilages. It inserts onto the medial border and coracoid process of the scapula, pulling it anteriorly and inferiorly on the thoracic wall. In addition, the muscle also stabilizes the scapula on the thoracic wall. Pectoralis minor is innervated by the medial pectoral nerve (C8-T1). The muscle can also be involved in pectoralis minor syndrome.

Serratus anterior

Serratus anterior is another large, fan-shaped muscle. It lies on the lateral aspect of the thorax, deep to the scapula and the pectoralis major and minor muscles. It consists of three parts:

• Superior part - originates from ribs 1-2 and the intercostal fascia. It inserts onto the anterior and posterior surfaces of the superior angle of the scapula.
• Middle part - originates from ribs 3-6 and inserts onto the anterior surface of the medial border of the scapula.
• Inferior part - originates from ribs 7-8 (sometimes 9 and 10). It inserts onto the anterior and posterior surfaces of the inferior angle of the scapula.

Serratus anterior is important in attaching the scapula to the thoracic wall. The muscle pulls the scapula in an anterolateral direction along the thoracic wall. Serratus anterior also rotates the scapula by pulling its inferior angle laterally. The muscle is innervated by the long thoracic nerve (C5-C7), which is a branch of the brachial plexus.

The innervation for the serratus anterior is very easy to remember if you just know the right mnemonics! 'SALT' stands for 'Serratus Anterior = Long Thoracic' and will help you remember the name of the nerve, while knowing 'C5, 6, 7 raise your arms to heaven!' means you'll never forget the nerve roots associated with it!

Subclavius

Subclavius is a small, round muscle found inferior to the clavicle. It originates from the sternal end of the first rib and its costal cartilage. It inserts onto the middle third of the clavicle on its anteroinferior aspect. By acting on the sternoclavicular joint, the subclavius anchors and depresses the clavicle. The muscle also protects the subclavian vessels and the brachial plexus which pass deep to it. The latter innervates the subclavius.

Intercostal muscles

The next set of torso muscles are found in the intercostal spaces between the ribs. The intercostal muscles consist of a group of three layered muscles, from superficial to deep: external, internal and innermost intercostals.

The fibres of the external intercostal muscles originate from the inferior border of the ribs. They extend in an inferomedial direction to the superior border of the rib below. The internal intercostal muscles extend in an inferoposterior direction from the costal groove of the ribs until the superior border of the rib below.  The innermost intercostals also originate from the costal groove of the ribs and insert onto the superior border of the rib below.

Collectively, the intercostal muscles support the intercostal spaces and thoracic cage. However, they also have additional individual functions. The external intercostals elevate the ribs during forced inspiration, expanding the thorax and lungs. In contrast, the internal and innermost intercostals depress the rib cage during forced expiration. This decreases the size of the thorax and lungs. All intercostal muscles are innervated by the intercostal nerves (T1-T11). These nerves run in the intercostal groove between the internal and innermost intercostal muscles.

Subcostal muscles

The subcostal muscles are strips of muscle located on the internal surface of the lower ribs, sharing a plane with the innermost intercostals. They extend from the internal surface of the lower ribs to the internal surface of a rib located 2-3 levels below. They support the intercostal spaces and thoracic cage, and depress the ribs during forced expiration. They receive innervation from the intercostal nerves.

Transversus thoracis

The transversus thoracis muscle is a thin, radiating muscle located on the internal aspect of the anterior thoracic wall. It originates from the inferoposterior surface of the sternum and xiphoid process. In turn, the muscle inserts on the internal surface of the 2nd to 6th costal cartilages. Transversus thoracis is a weak depressor of the rib cage, assisting expiration. It is innervated by the intercostal nerves.

Deepen your knowledge about the muscles of the thoracic wall with our additional learning materials.

Diaphragm

The diaphragm is a large, unpaired, dome shaped muscle located within the trunk. It separates the thoracic and abdominal cavities and facilitates the passage of anatomical structures via openings called hiatuses. The diaphragm consists of three parts:  

• Sternal part -originates from the posterior aspect of xiphoid process.
• Costal part - originates from the internal surfaces of the lower costal cartilages and ribs 7-12.
• Lumbar part - originates from the bodies of vertebrae L1-L3 and intervertebral discs, the anterior longitudinal ligament and the arcuate ligaments (medial, lateral).

All fibres converge on a central tendon in the middle of the trunk, which has no bony insertions. The diaphragm is the main muscle of breathing, responsible for inspiration. It also depresses the costal cartilages and receives innervation from the phrenic nerve (C3-C5).

Here is a collection of resources to improve your understanding of the diaphragm:

Muscles of the abdominal wall

We’ve finished with the muscles of the thoracic cage, so let’s descend on the anterior trunk to learn about the muscles of the abdominal wall. There are five muscles that form the abdominal part of the anterior trunk. These are the rectus abdominis, pyramidalis, external abdominal oblique, internal abdominal oblique and transversus abdominis. The first three are classified as vertical muscles and they are located near the midline. The remaining ones are flat muscles and they are located more laterally.

Together, these muscles form the anterolateral boundary of the abdominal cavity. They are involved in movements of the trunk, mainly flexion and rotation. They also contribute to changing the intra-abdominal pressure, which is essential for basic bodily functions such as micturition and defecation.

Rectus abdominis

Rectus abdominis is a long, strap-like, paired muscle that passes vertically over the entire length of the abdomen. The muscle originates from the pubic crest and pubic symphysis. It inserts onto the xiphoid process of the sternum and costal cartilages of the 5th to 7th ribs. 

Rectus abdominis acts as a flexor of the trunk and an accessory muscle of expiration. It also compresses the abdominal contents to protect them. Rectus abdominis can sometimes be seen beneath the skin in athletic individuals, resulting in what’s commonly referred to as a ‘six-pack’. It receives innervation via the intercostal and subcostal nerves.

Pyramidalis

Pyramidalis is a small triangular muscle found in the lower abdomen. It lies anterior to the rectus abdominis muscle. It extends from the pubic symphysis and pubic crest to the linea alba. The action of this muscle is to tense the linea alba, which is initiated by the subcostal nerve. Pyramidalis is a variable muscle of the abdominal wall, being absent in about 20% of the population.

Abdominal oblique muscles

It’s time to take a look at the three flat muscles of the anterolateral abdominal wall. The first two are the abdominal oblique muscles. These include the external abdominal oblique and the internal oblique muscles. The former are the most superficial, overlying the latter.

The external oblique muscles originate from the external surfaces of ribs 5-12. They insert onto the anterior half of the iliac crest, pubic tubercle and linea alba.
The internal oblique muscles originate from the anterior two-thirds of the iliac crest, the iliopectineal arch and the thoracolumbar fascia. Their insertion is onto the inferior borders of ribs 10-12, linea alba, the junction with cremaster muscle and the pectineal line of pubis (via conjoint tendon). The functions of the abdominal oblique muscles involve trunk flexion and ipsilateral rotation, as well as compression of abdominal viscera. They also assist with expiration. 

The abdominal oblique muscles are innervated by the intercostal, subcostal (T7-T12) and iliohypogastric nerves (L1). The internal oblique muscles are also innervated by the ilioinguinal nerve (L1).  

Transversus abdominis

The final muscle in the abdominal area of the anterior trunk is the transversus abdominis muscle. It lies underneath the internal oblique muscle, hence it is the deepest flat muscle. Transversus abdominis has an extensive origin from the costal cartilages of ribs 7-12, thoracolumbar fascia, anterior two thirds of the iliac crest and the iliopectineal arch. It crosses the abdomen in a horizontal direction to insert onto the linea alba, aponeurosis of the internal abdominal oblique muscle, pubic crest and pectineal line of the pubis. 

Contraction of transversus abdominis compresses the abdominal organs, assists expiration and causes ipsilateral trunk rotation. The muscle is innervated by the intercostal (T7-T11), subcostal (T12), iliohypogastric (L1) and ilioinguinal nerves (L1).

Quadratus lumborum

Quadratus lumborum is actually a muscle of the posterior wall, but it is often described as part of the ventral trunk musculature. This muscle originates from the iliac crest and iliolumbar ligament. It inserts onto the inferior border of the 12th rib. As it ascends, it also attaches to the transverse processes of L1-L4 vertebrae. 

Quadratus lumborum is involved in extending the trunk and flexing it laterally to the ipsilateral side. These actions are possible due to inputs provided by the subcostal nerve and the anterior rami of the L1-L4 spinal nerves.

Want to learn more about the muscles of the abdominal wall? Have a look at the study unit or test your knowledge with our custom quiz!

Superficial muscles of the back

For descriptive purposes, the muscles of the back are divided into two groups; superficial (extrinsic) muscles which move the upper limb and deep (intrinsic) muscles which act on the trunk. 

We will start by describing the superficial muscles of the back. They are arranged into two layers: 

• Superficial layer containing the trapezius, latissimus dorsi, rhomboid major and minor and levator scapulae muscles. 
• Intermediate layer possessing serratus posterior superior and inferior muscles.

Trapezius

Trapezius is a large, paired, triangular shaped muscle located in the upper back and neck. The muscle consists of three parts which fan out during their course: 

• Descending part - originates from the medial third of the superior nuchal line, the external occipital protuberance, nuchal ligament and the cervical vertebrae. It inserts into the lateral third of the clavicle. This part of the trapezius supports and draws the scapula superiomedially. In addition, it extends, rotates (contralateral) and laterally flexes (ipsilateral) the head and neck. 
• Transverse part - extends from the spinous processes of vertebrae T1-T4 (or C7-T3) to the acromion and superior crest of the spine of the scapula. This part supports and draws the scapula medially.
• Ascending part - originates from the spinous processes of vertebrae T5-T12 (or T2-T12). It inserts into the medial end of the spine of the scapula. The ascending part supports and moves the scapula inferomedially.

Trapezius receives motor innervation from the accessory nerve (CN XI). It also receives sensory innervation through the anterior rami of spinal nerves C3-C4 (via cervical plexus).

Latissimus dorsi

Latissimus dorsi is an expansive muscle located in the lower region of the back.  It overlies all the back muscles except for trapezius. Latissimus dorsi consists of four parts: 

• Vertebral part - originates from the spinous processes of vertebrae T7-T1 and the thoracolumbar fascia.
• Iliac part - extends from the posterior third of the iliac crest.
• CostaI part - originates from ribs 9-12.
• Scapular part - begins at the inferior angle of the scapula.

All fibres extend superiorly into the axilla to insert onto the medial lip of the intertubercular sulcus of the humerus. Latissimus dorsi has a variety of actions on the arm, including internal rotation, adduction and extension. It is also an accessory muscle of inspiration and expiration. Latissimus dorsi is innervated by the thoracodorsal nerve (C6-C8).

Rhomboids

Rhomboid major and rhomboid minor are a group of small, cylindrical muscles found between the vertebral column and the medial border of the scapula. Rhomboid major extends from the spinous process of vertebrae T2-T5 until the medial border of the scapula. Rhomboid minor originates from the nuchal ligament and spinous processes of C7-T1 vertebrae. It inserts at the root (medial end) of the spine of scapula.

Their main roles are to support and move the scapula superomedially. They also rotate the glenoid cavity inferiorly. The rhomboids are innervated by the dorsal scapular nerve (C5).

Levator scapulae

Levator scapulae is located deep to the trapezius and superior to the rhomboid minor muscle.  It originates from the transverse processes of C1-C4 vertebrae and attaches to the medial border of the scapula.. As the name suggests (‘elevator’), levator scapulae primarily elevates the scapula superomedially. It also rotates the glenoid cavity inferiorly and laterally flexes the neck (ipsilaterally). Levator scapulae is innervated by the dorsal scapular nerve (C5) and anterior rami of C3-C4 spinal nerves.

Serratus posterior

Diving deeper underneath all the previous superficial extrinsic muscles, one reaches the intermediate layer. he serratus posterior muscles are two oblique muscles:

• Serratus posterior superior - originates from the nuchal ligament and spinous processes of vertebrae C7-T3. It inserts on the superior borders of ribs 2-5, elevating them. It is innervated by the 2nd to 5th intercostal nerves.
• Serratus posterior inferior - begins at the spinous processes of vertebrae T11-L2 and inserts onto the inferior borders of ribs 9-12. This muscle depresses the ribs under the control of the 9th to 11 intercostal nerves and the subcostal nerve.

Learn more about the superficial muscles of the back with our learning materials that help you expand and test your knowledge in no time.

Deep muscles of the back

The deep (intrinsic) muscles of the back are a large group of muscles responsible for maintaining posture and controlling head and spine movements. They extend along the vertebral column from the pelvis all the way to the skull. They can be grouped into four layers: 

• Superficial - spinotransversales muscle (splenius capitis, splenius cervicis) 
• Intermediate - erector spinae (iliocostalis, longissimus, spinalis)
• Deep - transversospinal muscles (semispinalis, multifidus, rotatores longus and brevis) 
• Deepest - interspinales, intertransversarii and levatores costarum 

Superficial layer

The superficial layer of intrinsic back muscles is made up of the spinotransversales muscles, which are splenius capitis and splenius cervicis. They are located on the anterolateral aspects of the neck.

Splenius capitis is a deep, broad muscle found in the floor of the posterior triangle of the neck. It originates from the spinous processes of C7-T3 vertebrae and the nuchal ligament that covers them. It inserts onto the occipital bone (lateral part of the superior nuchal line) and the temporal bone (mastoid process) of the skull. Splenius capitis extends, laterally flexes and rotates the head ipsilaterally. It is innervated by lateral branches of posterior rami of spinal nerves C2-C3.

Splenius cervicis is the other muscle in the superficial layer. It originates below splenius capitis from the spinous processes of T3-T6 vertebrae. It extends superiorly and inserts onto the transverse processes of C1-C3. It has similar functions as splenius capitis: extension, lateral flexion and rotation of the neck. This muscle is innervated by posterior rami branches which stem from spinal nerves below C3.

Intermediate layer

The intermediate layer contains the massive erector spinae muscles. They lie in a groove on either side of the vertebral column in the space between the spinous processes of the vertebrae and the angles of the ribs. The erector spinae group contains three muscles, located from lateral to medial, as follows: iliocostalis, longissimus and spinalis. 

Iliocostalis forms the lateral column of erector spinae. It is divided into three regional areas or parts:

• Iliocostalis cervicis - extends from the angle of ribs 3-6 until the transverse processes of C4-C6 vertebrae.
• Iliocostalis thoracis - originates from the angle of ribs 7-12. It inserts onto the angles of ribs 1-6 and the transverse process of vertebra C7.
• Iliocostalis lumborum - originates from the lateral crest of sacrum, medial end of the iliac crest and the thoracolumbar fascia. It inserts onto the angle of ribs 5-12, transverse processes of vertebrae L1-L4 and the adjacent thoracolumbar fascia.

Contraction of iliocostalis produces extension and lateral flexion (ipsilateral) of the spine. 

Longissimus represents the intermediate column of erector spinae. It is also the largest member of the muscle group. Longissimus is divided into several regions or parts:

• Longissimus capitis - originates from the transverse processes of vertebrae C4-T5 and inserts onto the mastoid process of the temporal bone.
• Longissimus cervicis - attaches to the transverse processes of T1-T5 and transverse processes of C2-C6.
• Longissimus thoracis - originates from the medial end of the iliac crest, lateral crest of the sacrum and the spinous and transverse processes of L1-L5. It inserts onto the transverse processes of T1-T12 and the angles of ribs 5-12.

As a whole, longissimus extends and laterally flexes (ipsilateral) the spine. Longissimus capitis specifically performs the same movements, but on the head instead. The latter also rotates the head ipsilaterally.

Spinalis is the last and most medial muscle column that makes up the erector spinae. It is divided into two parts:

• Spinalis cervicis - originates from the spinous processes of C7-T1 and the nuchal ligament. The muscle inserts onto the spinous processes of C2-C4.
• Spinalis thoracis - originates from the spinous processes of T11-L2 and attaches onto the spinous processes of T2-T8.

Contraction of the spinalis muscles extends and laterally flexes the thoracic and cervical spine. All iliocostalis muscles are innervated by lateral branches of the posterior rami of the spinal nerves.

Mnemonic

A really easy way to remember the muscles in the intermediate layer is to learn the mnemonic 'I Like Standing'. It stands for:

(Lateral to medial)
Iliocostalis
Longissimus
Spinalis

Deep layer

The deep layer of back muscles are a group called the transversospinalis muscles. From superficial to deep, these include semispinalis, multifidus and rotatores. These muscles are located underneath the erector spinae, occupying the space between the spinous and transverse processes of the spine.

Semispinalis is the most superficial of the three muscles. This muscle is divided into three parts: 

• Semispinalis capitis - originates from the articular processes of vertebrae C4-C7 and the transverse processes of vertebrae T1-T6. It inserts between the superior and inferior nuchal lines of the occipital bone. 
• Semispinalis cervicis - attaches to the transverse processes of vertebrae T1-T6 and spinous processes of vertebrae C2-C5.
• Semispinalis thoracis - originates from the transverse processes of vertebrae T6-T10 and inserts onto the spinous processes of vertebrae C6-T4.

Semispinalis muscles act on the head, cervical and thoracic spines. They perform extension, rotation and lateral flexion (ipsilateral) of these structures. Semispinalis capitis is innervated by descending branches of the greater occipital (C2) and spinal nerves (C3). Semispinalis cervicis and thoracis are innervated by medial branches of the posterior rami of spinal nerves. 

Multifidus is a very thin muscle situated underneath semispinalis. This muscle spans a number of vertebrae to attach to the spinous processes of vertebrae in higher regions. It is divided into three parts:

• Multifidus cervicis - originates from the superior articular processes of vertebrae C4-C7.
• Multifidus thoracis - originates from the transverse processes of the thoracic vertebrae.
• Multifidus lumborum - originates from the mammillary processes of the lumbar vertebrae, posterior aspect of the sacrum, posterior superior iliac spine (PSIS) of ilium and the posterior sacroiliac ligament.

The fibres of all the parts of multifidus extend superiorly to insert onto the lateral aspect and tips of the spinous processes of vertebrae 2-5 levels above their origin. The functions of multifidus involve extension, rotation (contralateral) and lateral flexion (ipsilateral) of the spine. It is innervated by medial branches of the posterior rami of the spinal nerves.

The final group of muscles in the deep layer are the rotatores muscles. They are located underneath the multifidus and are most developed in the thoracic region. The rotatores has two parts:

• Rotatores brevis - originates from the transverse processes of vertebrae T2-T12 and inserts onto the laminae/spinous process of a vertebra one level above its origin.
• Rotatores longus - extends from the transverse processes of the thoracic vertebrae until the laminae/spinous process of a vertebra two levels above its origin.

Contraction of multifidus produces extension and contralateral rotation of the thoracic spine. The stimulus for this movement comes through the medial branches of the posterior rami of spinal nerves.

Deepest layer

The last and deepest layer of the intrinsic muscles of the back is composed of interspinales, intertransversarii and levatores costarum muscles. 

The interspinales muscles connect the spinous processes of adjacent vertebrae. They are divided into three regional parts:

• Interspinales cervicis - originates from the superior aspect of spinous processes of vertebrae C2-C7 and inserts onto the inferior aspect of spinous processes of vertebrae C1-C6.
• Interspinales thoracis has a variable origin from the superior aspect of the spinous processes of vertebrae T2, T11 and T12. The insertion is onto the inferior aspect of spinous processes of vertebrae T1, T10 and T11.
• Interspinales lumborum - originates from the superior aspects of the spinous processes of vertebrae L2-L5  and inserts onto the inferior aspects of the spinous processes of vertebrae L1-L4.

Interspinales extend the cervical and lumbar spine. They are innervated by the posterior rami of spinal nerves.

The intertransversarii are similar to the interspinales muscles, but connect adjacent transverse processes rather than spinous processes. There are four groups of these muscles; anterior cervical intertransversarii, posterior cervical intertransversarii, intertransversarii laterales lumborum and intertransversarii mediales lumborum.

They originate from the transverse processes of cervical and lumbar vertebrae and insert onto the transverse process of an adjacent vertebra. These muscles assist in lateral flexion of the spine and also work to stabilize the spine. They are innervated by the posterior and anterior rami of spinal nerves.

The last muscles in the deepest layer are levatores costarum. These muscles extend inferiorly from the transverse processes of the C7-T11 vertebrae to the superior border of the rib located one level below. They elevate the ribs and rotate the thoracic spine. They are innervated by posterior rami of spinal nerves T1-T12. 

You can use the following quiz to expand and test your knowledge about the deep muscles of the back.