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10.5: Axial Muscles of the Abdominal Wall and Thorax

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    By the end of this section, you will be able to:

    • Identify the intrinsic skeletal muscles of the back and neck, and the skeletal muscles of the abdominal wall and thorax
    • Identify the movement and function of the intrinsic skeletal muscles of the back and neck, and the skeletal muscles of the abdominal wall and thorax

    It is a complex job to balance the body on two feet and walk upright. The muscles of the vertebral column, thorax, and abdominal wall extend, flex, and stabilize different parts of the body’s trunk. The deep muscles of the core of the body help maintain posture as well as carry out other functions. The brain sends out electrical impulses to these various muscle groups to control posture by alternate contraction and relaxation. This is necessary so that no single muscle group becomes fatigued too quickly. If any one group fails to function, body posture will be compromised.

    Muscles of the Abdomen

    There are four pairs of abdominal muscles that cover the anterior and lateral abdominal region and meet at the anterior midline. These muscles of the anterolateral abdominal wall can be divided into four groups: the external obliques, the internal obliques, the transversus abdominis, and the rectus abdominis (Figure \(\PageIndex{1}\) and Table \(\PageIndex{1}\)).

    Anterior lateral view of muscles of the abdomen; anterior view of the posterior muscles of the abdomen
    Figure \(\PageIndex{1}\): Muscles of the Abdomen. (a) The anterior abdominal muscles include the medially located rectus abdominis, which is covered by a sheet of connective tissue called the rectus sheath. On the flanks of the body, medial to the rectus abdominis, the abdominal wall is composed of three layers. The external oblique muscles form the superficial layer, while the internal oblique muscles form the middle layer, and the transverses abdominus forms the deepest layer. (b) The muscles of the lower back move the lumbar spine but also assist in femur movements. (Image credit: "Muscles of the Abdomen" by Openstax is licensed under CC BY 4.0)
    Table \(\PageIndex{1}\): Muscles of the Abdomen
    Prime Mover Origin Insertion Movement Target Target motion direction
    External obliques; Internal obliques Ilium; ribs 5 - 12 Ilium; linea alba; ribs 7 - 10 Twisting at waist; also bending to the side Vertebral column Supination; lateral flexion
    Quadratus lumborum Ilium; ribs 5 - 10 Rib 12; vertebrae L1 - L4 Bending to the side Vertebral column Lateral flexion
    Rectus abdominis Pubis Ribs 5 and 7; sternum Sitting up Vertebral column Flexion
    Transverse abdominus Ilium; ribs 5 - 10 Linea alba; pubis; sternum Squeezing abdomen during forceful exhalations, defecation, urination, and childbirth Abdominal cavity Compression

    There are three flat skeletal muscles in the antero-lateral wall of the abdomen. The external oblique, closest to the surface, extend inferiorly and medially, in the direction of sliding one’s four fingers into pants pockets. Perpendicular to it is the intermediate internal oblique, extending superiorly and medially, the direction the thumbs usually go when the other fingers are in the pants pocket. The deep muscle, the transversus abdominis, is arranged transversely around the abdomen, similar to the front of a belt on a pair of pants. This arrangement of three bands of muscles in different orientations allows various movements and rotations of the trunk. The three layers of muscle also help to protect the internal abdominal organs in an area where there is no bone.

    The linea alba is a white, fibrous band that is made of the bilateral rectus sheaths that join at the anterior midline of the body. These enclose the rectus abdominis muscles (a pair of long, linear muscles, commonly called the “sit-up” muscles) that originate at the pubic crest and symphysis, and extend the length of the body’s trunk. Each muscle is segmented by three transverse bands of collagen fibers called the tendinous intersections. This results in the look of “six-pack abs,” as each segment hypertrophies on individuals at the gym who do many sit-ups.

    The posterior abdominal wall is formed by the lumbar vertebrae, parts of the ilia of the hip bones, psoas major and iliacus muscles, and quadratus lumborum muscle. This part of the core plays a key role in stabilizing the rest of the body and maintaining posture.


    Physical Therapist

    Those who have a muscle or joint injury will most likely be sent to a physical therapist (PT) after seeing their regular doctor. PTs have a master’s degree or doctorate, and are highly trained experts in the mechanics of body movements. Many PTs also specialize in sports injuries.

    If you injured your shoulder while you were kayaking, the first thing a physical therapist would do during your first visit is to assess the functionality of the joint. The range of motion of a particular joint refers to the normal movements the joint performs. The PT will ask you to abduct and adduct, circumduct, and flex and extend the arm. The PT will note the shoulder’s degree of function, and based on the assessment of the injury, will create an appropriate physical therapy plan.

    The first step in physical therapy will probably be applying a heat pack to the injured site, which acts much like a warm-up to draw blood to the area, to enhance healing. You will be instructed to do a series of exercises to continue the therapy at home, followed by icing, to decrease inflammation and swelling, which will continue for several weeks. When physical therapy is complete, the PT will do an exit exam and send a detailed report on the improved range of motion and return of normal limb function to your doctor. Gradually, as the injury heals, the shoulder will begin to function correctly. A PT works closely with patients to help them get back to their normal level of physical activity.

    Muscles of the Thorax

    The muscles of the chest serve to facilitate breathing by changing the size of the thoracic cavity (Table \(\PageIndex{2}\)). When you inhale, your chest rises because the cavity expands. Alternately, when you exhale, your chest falls because the thoracic cavity decreases in size.

    Table \(\PageIndex{2}\): Muscles of the Thorax
    Prime Mover Origin Insertion Movement Target Target motion direction
    Diaphragm Lumbar vertebrae; ribs 6 - 12; sternum Central tendon Inhalation; exhalation Thoracic cavity Compression; expansion
    External intercostals Rib superior to each intercostal muscle Rib inferior to each intercostal muscle Inhalation; exhalation Ribs Elevation (expands thoracic cavity)
    Internal intercostals Rib inferior to each intercostal muscle Rib superior to each intercostal muscle Forced exhalation Ribs Movement along superior/inferior axis to bring ribs closer together

    The Diaphragm

    The change in volume of the thoracic cavity during breathing is due to the alternate contraction and relaxation of the diaphragm (Figure \(\PageIndex{2}\)). It separates the thoracic and abdominal cavities, and is dome-shaped at rest. The superior surface of the diaphragm is convex, creating the elevated floor of the thoracic cavity. The inferior surface is concave, creating the curved roof of the abdominal cavity.

    Inferior view of the diaphragm
    Figure \(\PageIndex{2}\): Muscles of the Diaphragm. The diaphragm separates the thoracic and abdominal cavities. (Image credit: "The Diaphragm" by Openstax is licensed under CC BY 4.0)

    Defecating, urination, and even childbirth involve cooperation between the diaphragm and abdominal muscles (this cooperation is referred to as the “Valsalva maneuver”). You hold your breath by a steady contraction of the diaphragm; this stabilizes the volume and pressure of the peritoneal cavity. When the abdominal muscles contract, the pressure cannot push the diaphragm up, so it increases pressure on the intestinal tract (defecation), urinary tract (urination), or reproductive tract (childbirth).

    The inferior surface of the pericardial sac and the inferior surfaces of the pleural membranes (parietal pleura) fuse onto the central tendon of the diaphragm. To the sides of the tendon are the skeletal muscle portions of the diaphragm, which insert into the tendon while having a number of origins including the xiphoid process of the sternum anteriorly, the inferior six ribs and their cartilages laterally, and the lumbar vertebrae and 12th ribs posteriorly.

    The diaphragm also includes three openings for the passage of structures between the thorax and the abdomen. The inferior vena cava passes through the caval opening, and the esophagus and attached nerves pass through the esophageal hiatus. The aorta, thoracic duct, and azygous vein pass through the aortic hiatus of the posterior diaphragm.

    The Intercostal Muscles

    There are three sets of muscles, called intercostal muscles, which span each of the intercostal spaces. The principal role of the intercostal muscles is to assist in breathing by changing the dimensions of the rib cage (Figure \(\PageIndex{3}\)).

    Anterior view of the intercostal muscles of the thorax; Anterior view of muscles found between two ribs
    Figure \(\PageIndex{3}\): Intercostal Muscles. The external intercostals are located laterally on the sides of the body. The internal intercostals are located medially near the sternum. The innermost intercostals are located deep to both the internal and external intercostals. (Image credit: "The Thorax" by Openstax is licensed under CC BY 4.0)

    The 11 pairs of superficial external intercostal muscles aid in inspiration of air during breathing because when they contract, they raise the rib cage, which expands it. The 11 pairs of internal intercostal muscles, just under the externals, are used for expiration because they draw the ribs together to constrict the rib cage. The innermost intercostal muscles are the deepest, and they act as synergists for the action of the internal intercostals.

    Muscles of the Pelvic Floor and Perineum

    The pelvic floor is a muscular sheet that defines the inferior portion of the pelvic cavity. The pelvic diaphragm, spanning anteriorly to posteriorly from the pubis to the coccyx, comprises the levator ani and the ischiococcygeus. Its openings include the anal canal and urethra, and the vagina in females.

    The large levator ani consists of two skeletal muscles, the pubococcygeus and the iliococcygeus (Figure \(\PageIndex{4}\)). The levator ani is considered the most important muscle of the pelvic floor because it supports the pelvic viscera. It resists the pressure produced by contraction of the abdominal muscles so that the pressure is applied to the colon to aid in defecation and to the uterus to aid in childbirth (assisted by the ischiococcygeus, which pulls the coccyx anteriorly). This muscle also creates skeletal muscle sphincters at the urethra and anus.

    Inferior view of muscles of the pelvic floor
    Figure \(\PageIndex{4}\): Muscles of the Pelvic Floor. The pelvic floor muscles support the pelvic organs, resist intra-abdominal pressure, and work as sphincters for the urethra, rectum, and vagina. (Image credit: "Muscles of the Pelvic Floor" by Openstax is licensed under CC BY 4.0)

    The perineum is the diamond-shaped space between the pubic symphysis (anteriorly), the coccyx (posteriorly), and the ischial tuberosities (laterally), lying just inferior to the pelvic diaphragm (levator ani and coccygeus). Divided transversely into triangles, the anterior is the urogenital triangle, which includes the external genitals. The posterior is the anal triangle, which contains the anus (Figure \(\PageIndex{5}\)). The perineum is also divided into superficial and deep layers with some of the muscles common to males and females (Table \(\PageIndex{3}\)). Females also have the compressor urethrae and the sphincter urethrovaginalis, which function to close the vagina. In males, there is the deep transverse perineal muscle that plays a role in ejaculation.

    Inferior views of the male and female perineal muscles
    Figure \(\PageIndex{5}\): Muscles of the Perineum. The perineum muscles play roles in urination in both sexes, ejaculation in men, and vaginal contraction in women. (Image credit: "Muscles of the Perineum" by Openstax is licensed under CC BY 4.0)
    Table \(\PageIndex{3}\): Muscles of the Perineum Common to Males and Females
    Prime Mover Origin Insertion Movement Target Target Motion Direction
    Levator ani; pubococcygeus; levator ani iliococcygeus Pubis; ischium Urethra; anal canal; perineal body; coccyx Defecation; urination; birth; coughing Abdominal cavity Superior (resists pressure during abdominal compression)
    Superficial Muscles
    Bulbospongiosus Perineal body Perineal membrane; corpus spongiosum of penis; deep fascia of penis; clitoris in female Involuntary response that compresses urethra when excreting urine in both sexes or while ejaculating in males; also aids in erection of penis in males Urethra Compression
    Ischiocavernosus Ischium; ischial rami; pubic rami Pubic symphysis; corpus cavernosum of penis in male; clitoris of female Compresses veins to maintain erection of penis in males; erection of clitoris in females Veins in penis and clitoris Compression
    Superficial transverse perineal Iscium Perineal body None - supports perineal body maintaining anus at center of perineum Perineal body none
    Deep Muscles
    External anal sphincter Anoccoccygeal ligament Perineal body Closes anus Anus Sphincter
    External urethral sphincter Ischial rami; pubic rami

    Male: median raphe

    Female: vaginal wall

    Voluntarily compresses urethra during urination Urethra Compression

    Concept Review

    Made of skin, fascia, and four pairs of muscle, the anterior abdominal wall protects the organs located in the abdomen and moves the vertebral column. These muscles include the rectus abdominis, which extends through the entire length of the trunk, the external oblique, the internal oblique, and the transversus abdominus. The quadratus lumborum forms the posterior abdominal wall.

    The muscles of the thorax play a large role in breathing, especially the dome-shaped diaphragm. When it contracts and flattens, the volume inside the pleural cavities increases, which decreases the pressure within them. As a result, air will flow into the lungs. The external and internal intercostal muscles span the space between the ribs and help change the shape of the rib cage and the volume-pressure ratio inside the pleural cavities during inspiration and expiration.

    The perineum muscles play roles in urination in both sexes, ejaculation in men, and vaginal contraction in women. The pelvic floor muscles support the pelvic organs, resist intra-abdominal pressure, and work as sphincters for the urethra, rectum, and vagina.

    Review Questions

    Q. Which of the following abdominal muscles is not a part of the anterior abdominal wall?

    A. quadratus lumborum

    B. rectus abdominis

    C. interior oblique

    D. exterior oblique


    Answer: A

    Q. Which muscle pair plays a role in respiration?

    A. intertransversarii, interspinales

    B. semispinalis cervicis, semispinalis thoracis

    C. trapezius, rhomboids

    D. diaphragm, scalene


    Answer: D

    Q. What is the linea alba?

    A. a small muscle that helps with compression of the abdominal organs

    B. a long tendon that runs down the middle of the rectus abdominis

    C. a long band of collagen fibers that connects the hip to the knee

    D. another name for the tendinous inscription


    Answer: B

    Critical Thinking Questions

    Q. Describe the fascicle arrangement in the muscles of the abdominal wall. How do they relate to each other?


    A. Arranged into layers, the muscles of the abdominal wall are the internal and external obliques, which run on diagonals, the rectus abdominis, which runs straight down the midline of the body, and the transversus abdominis, which wraps across the trunk of the body.

    Q. What are some similarities and differences between the diaphragm and the pelvic diaphragm?


    A. Both diaphragms are thin sheets of skeletal muscle that horizontally span areas of the trunk. The diaphragm separating the thoracic and abdominal cavities is the primary muscle of breathing. The pelvic diaphragm, consisting of two paired muscles, the coccygeus and the levator ani, forms the pelvic floor at the inferior end of the trunk.


    anal triangle
    posterior triangle of the perineum that includes the anus
    caval opening
    opening in the diaphragm that allows the inferior vena cava to pass through; foramen for the vena cava
    compressor urethrae
    deep perineal muscle in women
    deep transverse perineal
    deep perineal muscle in men
    skeletal muscle that separates the thoracic and abdominal cavities and is dome-shaped at rest
    external intercostal
    superficial intercostal muscles that raise the rib cage
    external oblique
    superficial abdominal muscle with fascicles that extend inferiorly and medially
    muscle that makes up the levator ani along with the pubococcygeus
    innermost intercostal
    the deepest intercostal muscles that draw the ribs together
    intercostal muscles
    muscles that span the spaces between the ribs
    internal intercostal
    muscles the intermediate intercostal muscles that draw the ribs together
    internal oblique
    flat, intermediate abdominal muscle with fascicles that run perpendicular to those of the external oblique
    muscle that assists the levator ani and pulls the coccyx anteriorly
    levator ani
    pelvic muscle that resists intra-abdominal pressure and supports the pelvic viscera
    linea alba
    white, fibrous band that runs along the midline of the trunk
    pelvic diaphragm
    muscular sheet that comprises the levator ani and the ischiococcygeus
    diamond-shaped region between the pubic symphysis, coccyx, and ischial tuberosities
    muscle that makes up the levator ani along with the iliococcygeus
    quadratus lumborum
    posterior part of the abdominal wall that helps with posture and stabilization of the body
    rectus abdominis
    long, linear muscle that extends along the middle of the trunk
    rectus sheaths
    tissue that makes up the linea alba
    sphincter urethrovaginalis
    deep perineal muscle in women
    tendinous intersections
    three transverse bands of collagen fibers that divide the rectus abdominis into segments
    transversus abdominis
    deep layer of the abdomen that has fascicles arranged transversely around the abdomen
    urogenital triangle
    anterior triangle of the perineum that includes the external genitals

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