Muscles of the Torso

We have previously discussed many of the muscles found in the torso. The muscles previously discussed involve movement of the shoulders, arms, hips, and legs. We will mention these muscles in passing in this article, but will rely on earlier curriculum discussions to cover their details.

In this article we will discuss muscles found in the abdomen, chest, and back areas of the torso.

Abdominal Muscles

By OpenStax [CC BY 4.0 (https://creativecommons.org/licenses/by/4.0)], via Wikimedia Commons

The abdomen contains numerous muscles. When you think of these muscles you likely think of your ‘abs’, but these represent only some of the muscles found in your abdomen. In this section we will cover three different muscle groups found in the abdomen.

Anterior Abdominal Muscles

The anterior abdominal wall (anterior abdominal muscles) contains three separate layers of muscle. These are the muscles that are typically referred to as your ‘abs’. These muscles are found in layers. Moving from the deepest to the most superficial these muscles are the transversus abdominis, internal obliques, and external obliques. There are right and left instances of each of these muscles. Superficial to these muscles are the rectus abdominis muscles, which bridge the center line of the body.

The transversus abdominis originates from numerous surfaces including lateral portions of the inguinal ligament, the anterior and medial lip of the iliac crest, the cartilage of ribs six through twelve, and the thoracolumbar fascia. It insert at the center-line along the linea alba and inferiorly via a tendon (along with the internal obliques) at the crest and pectineal line of the pubis. Like the other flat muscles in the abdominal wall there is a transversus abdominis muscle on the left and right side of the abdomen.

This muscle has fibers that run generally horizontal when you stand erect. This means the fibers run in a transverse direction, which explains the name of the muscle.

There are numerous functions associated with the transversus abdominis. Perhaps the most important is general stabilization of the spine and abdominal area. The muscle is responsible from compressing the abdomen and therefore plays an important role in keeping the abdomen pulled inward, which helps maintain a flat abdomen.

Superficial to the transversus abdominis are the internal oblique muscles. The fibers on this muscle run obliquely superiormedially (upward and inward) from their origins along the thoracolumbar ligament of the lower back, the lateral portion of the inguinal ligament, and the anterior portion of the iliac crest. The muscle inserts along the inferior edges of ribs eight through twelve, the linea alba, and joins with the tendon from the transversus abdominis to insert at the crest and pectineal line of the pubis.

Functionally the internal oblique muscles help contract the lower abdomen during exhalation, aiding the respiratory process. The muscles also assist, along with the opposite side external obliques, with rotation of the torso. Contracting both the left internal oblique muscle and the right external oblique muscle will rotate the torso such that the right shoulder moves forward and inward toward the left hip. In addition the internal oblique muscles are responsible for lateral flexing of the spinal column, allowing you to flex your torso to the left or right.

The external oblique muscles are superficial to the internal oblique muscles. The external oblique muscles have muscle fibers oriented at 90° to the fibers of the internal oblique muscles. The fibers in the external oblique muscles run inferiorly and anteriorly from their origins on the external surfaces of rubs five through twelve. The muscle inserts at various locations. The lower fibers of the muscle insert at the outer anterior lip of the iliac crest. The remaining fibers insert at (and become part of) the aponeurosis of the abdominal external oblique muscle. This thin but strong membrane covers nearly the entirety of the front abdomen and joins both left and right external oblique muscles together to form the linea alba.

In Latin the word rectus means straight.  The rectus abdominis muscle is a superficial paired muscle that covers the entire center line of the abdomen. The left and right muscles connect at the mid-line to the linea alba. The muscle originates at the crest of the pubis bone. Its muscle fibers proceed straight superiorly and insert into the cartilage of ribs five through seven as well as at the xiphoid process of the sternum.  Tendinous fibers cross the muscle in a transverse manner at various points and thereby dividing the muscle into multiple segments or false bellies. These are the muscle sections that appear on a thin person who has a well-developed set of “six-pack” muscles. The muscle also connects to fibrous tissues originating from other muscles throughout the abdomen.

The rectus abdominis provides flexure of the lumbar spine. This allows you to do crunches and to rotate your pelvis toward your chest when doing certain floor exercises. The rectus abdominis also supports forceful exhalation and as occurs after heavy exercise. The muscle also helps to ensure the structural integrity of organs within the abdominal cavity.

Posterior Abdominal Muscles

A group of muscles form the posterior abdominal muscles found deep in the abdominal cavity. The muscles in this group include the iliacus, psoas major, iliopsoas, psoas minor, and the quadratus lumborum. Most of these muscles originate from the lumbar vertebrae as well as from the twelfth thoracic vertebrae and the associated twelfth rib. Many anatomists also include the diaphragm as a member of this muscle group.  We will include the diaphragm in our discussion.

We previously discussed the iliacus and psoas major muscles in some detail in the article on Muscles of the Hip and Thigh. That article also discusses that these two muscles join together to form the iliopsoas muscle. The primary function of these muscles is to flex the femur.

The psoas minor muscles originate along the body of the T12 and L1 vertebrae. The left and right muscles then insert into the iliopectineal eminence (the area in which the pubis and ilium join) and the pectineal line of the pubis bone. The two muscles function to flex and laterally bend the lumbar spine.

Originating along the posterior iliac crest and the iliolumbar ligament, the quadratus lumborum muscle rises superiorly and inserts along the inferior surface of the twelfth rib and the transverse processes of the first four lumbar vertebrae.  The muscle functions to elevate the ilium, flex the vertebral column laterally, fix the twelfth rib during exhalation, and extend the lumbar vertebral column.

By OpenStax [CC BY 4.0 (https://creativecommons.org/licenses/by/4.0)], via Wikimedia Commons

The diaphragm (or thoracic diaphragm) forms the superior boundary of the abdominal cavity and serves to separate the thoracic cavity from the abdominal cavity. When the diaphragm contracts it presses downward into the abdominal cavity, thereby increasing the size of the thoracic cavity and drawing air into the lungs.

The diaphragm is a large, dome-shaped skeletal muscle composed of left and right lobes and a central area supporting attachments to the spinal column. The right dome (or lobe) of the diaphragm usually sits slightly higher than the left lobe. Anatomists think that this occurs due to presence of the liver on the right side.

The diaphragm has attachments at many points in its vicinity.  In the central area of the diaphragm two large tendons insert into (or arise from) the vertebral column. These tendons are the left and right crus. The left crus arises from vertebrae L1-L2, the right crus arises from vertebrae L1-L3. The diaphragm also inserts at the xiphoid process of the sternum and along the costal margin of the rib cage. In addition the muscle inserts into ribs six through twelve. The diaphragm also arises from many other fibrous structures in the thorax, including the inferior portions of the pericardium.

The various fibers of the diaphragm converge along a large lateral central tendon (depicted in white as a large arc in the associated diagram). This represents the point of insertion for the muscle fibers. Breathing may cause the central tendon to pull downward, expanding the volume of the thoracic cavity. The means the central tendon moves inferiorly. Alternately, the central tendon can be held in a relatively fixed position when the diaphragm contracts. This causes the low rib cage to lift upward instead.

During inhalation the diaphragm commonly pushes the abdominal cavity inferiorly (pulling the central tendon in an inferior direction), thereby increasing the size of the thoracic cavity. This decreases the pressure in the thoracic cavity and causes the lungs to fill with air. This method of inhalation is commonly called stomach breathing and this occurs when the ribs are held in a relatively fixed position while the central tendon moves.

Chest breathing occurs when the ribs are not held in a fixed position and are instead allowed to expand through actions of the external intercostal muscles (discussed later) and the diaphragm. During chest breathing the central tendon of the diaphragm is held in a fixed position which helps pull the lower rib cage upward, applying outward pressure on the ribs. This again increases the volume of the thorax and reduces the pressure within the thoracic cavity, again allowing air to enter the lungs.

When the diaphragm relaxes the abdominal cavity expands superiorly (when stomach breathing) or the rib cage returns to its relaxed state (when chest breathing) causing a reduction in the size of the thoracic cavity. With the aid of the internal intercostal muscles (when chest breathing) the pressures within the thoracic cavity increase, resulting in exhalation.

In most cases some component of chest and stomach breathing occurs with each breath. In reality we are only speaking of a matter of degree when we say someone is chest or stomach breathing. What we mean is they are primarily utilizing chest or stomach breathing methods. There will almost always be some component of the other breathing method used as well. One other thing to point out. Whether you are using chest breathing or stomach breath, the diaphragm is always involved.

Because the diaphragm can push the abdomen in an inferior direction it can increase pressures within the abdominal cavity. This allows the diaphragm to assist with activities associated with increased abdominal pressures, such as vomiting, bowel elimination, and childbirth.

Organ Muscles

There are also a great many involuntary smooth muscles found in the organs within the abdominal cavity. The stomach, large intestine, and small intestine all have abundant smooth muscles that help churn and produce the wave action that moves chyme through the system. Numerous sphincters are also present that control the flow of chyme, bile salts, and enzymes throughout the digestive system.  We discussed most of these muscles at some length in earlier curriculum material on the digestive system.

Thorax Muscles

We have previously covered many of the muscles found in the chest and back in our article on Muscles Affecting the Upper Arm. We will not cover these muscles in additional detail in this section. We will cover muscles in the thorax that were not previously covered in other curriculum materials.

Intercostal Muscles

By OpenStax [CC BY 4.0 (https://creativecommons.org/licenses/by/4.0)], via Wikimedia Commons

The intercostal muscles reside in the intercostal space between two adjacent ribs. The muscles are found in three layers, with each layer providing a different function.

The most superficial of the intercostal muscles are the external intercostal muscles. Each external intercostal muscle originates on the inferior margins of one rib and inserts into the superior margins of the rib below. These muscles help expand the chest wall during inhalation. The muscle fibers run inferoanteriorly and lie deep to and continuous with the external oblique muscles.

The internal intercostal muscles originate along the inferior edge of one rib and insert into the superior edge of the rib below. Muscle fibers run generally perpendicular (inferoposteriorly) to those of the external intercostal muscles. Their function is to contract the chest wall during periods of strong exhalation.

Deep to the internal intercostal muscles lie the innermost intercostal muscles. These muscles originate on the medial superior edge of one rib and insert along the superior surface of the rib below. Muscle fibers generally run in the same direction as the internal intercostal muscles (not as depicted in the diagram).

Subcostal Muscles

Subcostal muscles are thin muscles that originate from the medial edge of one rib and insert along the superior medial border of the second or third rib below. Anatomists often suggest these muscles serve the same function as the internal intercostal muscles. The subcostal muscles reside primarily in the lower portions of the thoracic cage.

Transversus Thoracis Muscle

The transversus thoracis muscle sits on the posterior edges of the anterior rib cage adjacent to the sternum.  The view at right depicts these muscles when view from the perspective of the spine. The muscle originates along the posterior side of the sternum, the posterior of the xiphoid process, and from the ends of the costal cartilages near the sternum at ribs four through seven.  The muscle inserts along the posterior edges of ribs two through six.

This muscle has limited function. It weakly aids compression of the ribs during forced exhalation.

Back Muscles

The muscles found in the back or posterior of the torso are commonly categorized as superficial, intermediate, and intrinsic (or deep) muscles. We discussed many of the superficial muscles in the article on Muscles Affecting the Upper Arm.  While we may briefly mention a muscle described in the earlier article, we will rely on the earlier article for a detailed discussion for these muscles. You may wish to refer to the diagram at left for the location and size of the muscles discussed in this section.

By OpenStax [CC BY 4.0 (https://creativecommons.org/licenses/by/4.0)], via Wikimedia Commons

Superficial Back Muscles

The most superficial back muscles include the trapezius, latissimus dorsi, levator scapulae and the rhomboids. These muscles facilitate movement of the shoulder.

Trapezius Muscle

The trapezius muscle functions to elevate, rotate, and stabilize the scapula, particularly during abduction of the arm. The trapezius muscles play a major role when you shrug your shoulders. The fibers in the upper portions of the muscle are largely responsible for these actions. The lower (inferior) muscle fibers pull the scapula in an inferior direction.  The middle muscle fibers function to retract the scapula during adduction. The muscle also aids in rotation and tilting of the head and neck.

The trapezius are large and generally flat muscles (one on either side of the body) in the upper portions of the back as well as the shoulders and neck. It originates along the C7-T12 vertebrae and from the nuchal crest of the occipital bone of the skull.  It inserts along the clavicle, and both the acromion and spine of the scapula.

Latissimus Dorsi

The latissimus dorsi muscles are found inferior to the trapezius muscles. We discussed the latissimus dorsi muscles in some detail in the article on Muscles Affecting the Upper Arm. Please refer to that article for more details about this pair of muscles.

Levator Scapulae

The levator scapulae muscle bends the neck laterally when the scapula is held rigid. When the spine is held rigid the levator scapulae causes the scapula to pull upward and medially. The muscle often works with the rhomboid muscles (discussed next) and the trapezius muscles to help various movements of the neck and shoulders.

The levator scapulae originates along the transverse processes of the C1-C4 vertebrae. It inserts on the medial side of the scapula between the spine of the scapula and superior angle of the scapula.

Rhomboids

The rhomboid muscles consist of two separate muscles on each side of the back, the rhomboid major and rhomboid minor muscles.

The rhomboid minor helps to retract and rotate the scapula. The muscle originates along the spinous processes of the C7-T1 vertebrae and inserts on the medial edge of the scapula superior to the spine of the scapula. It sits superiorly to the rhomboid major muscle.

The rhomboid major muscle originates at the spinous processes of the T2-T5 vertebrae. It inserts along the medial edge of the scapula inferior to the spine of the scapula. The muscle functions to rotate and retract the scapula.

Intermediate Back Muscles

There are two intermediate back muscles that rest generally between the superficial and deep muscles. These are the serratus posterior superior and serratus posterior inferior muscles.

Serratus Posterior Superior

This muscle lies deep to the rhomboids. It originates along the C7-T3 vertebrae and from the lower portions of the ligamentum nuchae. The muscle then descends inferiorly and laterally and eventually attaches to ribs 2 through 5. It functions to lift these ribs to aid with deep inhalation.

Serratus Posterior Inferior

The serratus posterior inferior muscle lies deep to the latissimus dorsi muscle. It originates the T11 – L3 (sometimes L2) vertebrae and then extends superiorly and laterally before inserting into the inferior border of ribs 9 through 12.  The muscle aids with rotation of the trunk of the body and assists with forced exhalation.

Intrinsic Back Muscles

There are numerous muscles in the back that support movements of the thoracic cage. Many of these muscles also play a role in movements of the neck and head.

Iliocostalis

These long muscles reside deep on either side of the back and generally run parallel to the spinal column. The muscles consist of three separate segments, each affecting different movements of the neck and back. We will discuss each of these segments separately.

The iliocostalis cervicis muscles (one on either side of back) function to extend the vertebrae of the neck. When both muscles contract they cause you to raise your head.  If only one muscle contracts then your head will tilt laterally to that side of body. The muscles originate from the angles of ribs 3 – 6. They insert into the transverse processes of vertebrae C4-C6.

The iliocostalis throacis muscles (as with all iliocostalis muscles, one is found on each side of the back) originate along the angles of ribs 7-12. They insert at the angles of ribs 1 – 6 and the transverse process of vertebrae C7. When both muscles contract they function to extend the vertical column, allowing you to return to an erect posture after leaning slightly forward. If only one muscle contracts then your upper torso will tilt laterally toward that side of the body.

The iliocostalis lumborum muscles originate along the medial and lateral crests of the sacrum and the medial crest of the iliac. The muscles insert into ribs 6 – 12 lateral to the origins of the iliocostalis thoracis muscles. When used bilaterally the muscles extend the lower torso, facilitating raising your torso from a deep forward bow or lean. If only a single muscle flexes then a lateral lean originating at the lower vertical column results.

Longissimus

Like the iliocostalis muscles, the longissimus muscles are a set of three separate muscles that aid in large movements of the torso. Identical muscles reside on either side of the back. We will discuss each of these muscles in turn.

The longissimus capitis muscles originate at the transverse processes of vertebrae T1 – T5 and the articular processes of vertebrae C4 – C7. The muscle inserts along the posterior aspect of the mastoid process of the temporal bone of the skull. When both muscles contract they function to extend the head. If you stand erect and tilt your head backward (hyper-extending) your head then you are employing these muscles. If using only one of these muscles then the head will tilt and/or rotate to that side of the body.

The longissimus cervicis muscle extends and hyperextends the neck vertebrae. These muscles originate at the transverse process of vertebrae T1- T5 They insert into the transverse process of vertebrae C2-C6. These muscles work with the iliocostalis cervicis muscles. If only one muscle contracts then the neck vertebrae will tilt laterally toward that side of the body.

The longissimus thoracis muscles have many points of origin, including the transverse and spinous process of the lumbar vertebrae and the sacrum and iliac crests via the iliocostalis lumborum tendon and the lumbosacral aponeurosis connective tissues. They insert into the transverse process of vertebrae T1 – T12 and the tubercles and angles of ribs 4 – 12. Like other muscles in this group the longissimus thoracis works bilaterally to extend the torso and unilaterally to tilt the torso toward the side of the contracting muscle.

Multifarious

This long muscle originates from the posterior of the sacrum, the superior iliac spine, the mammillary processes of the lumbar vertebrae, transverse processes of the thoracic vertebrae and articular processes of vertebrae C4- C7. Muscles fibers originating from two to four vertebrae below connect to the spinous process of each vertebrae except C1. Bilaterally this muscle extends the torso. When employed unilaterally the muscle functions to tilt the torso toward that side of the body and/or to rotate the torso away from that side of the body (contralateral rotation).

Rotatores

There are eleven rotatores muscles on each side of the body. These small muscles lie deep to the multifarious muscles. Each muscle originates on the posterior and superior surfaces of one vertebrae and inserts into the lower border and lamina of the vertebrae above. These muscle are likely involved in maintaining posture. The muscles are also involved in forward and backward bending of the spine and aid with rotation of the torso.

Semispinalis

The semispinalis muscles represent another group of three muscles on each side of the back. These muscles provide functions similar to the earlier muscle groups we have discussed.

The splenius capitis originates from the spinous process of vertebrae C7 – T4. It also arises from the nuchal ligament in the neck. The muscle inserts along the mastoid process of the temporal bone in the skull. This flat muscle plays a significant role in head extension (during bilateral contractions) and lateral flexion and rotation of the cervical spine (during unilateral contractions).

Arising from the spinous process of vertebrae T3 – T6, the splenius cervicis muscle inserts at the posterior tubercles of the transverse process of vertebrae C1 – C3. The muscle (one on either side of the neck) functions to extend the cervical column. When used unilaterally the muscle also functions to laterally tilt and/or rotate the cervical spine toward the side of the contracting muscle.

Spinalis

Originating from the spinous process of vertebrae T10 – L3, the splenius thoracis muscle aids extension of the lower spinal column(during bilateral contraction). The muscle inserts into the spinous process of vertebrae T2 – T8 and aids with lateral tilt and rotation of the torso when employing unilateral muscle contractions.

 

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