Trunk Muscles

The trunk, or torso, is the central part of the human body and consists of the thorax, or chest, and the abdomen, the region between the thorax and pelvis. Movement in the thorax is driven by muscles that attach to bones, such as the ribs, sternum, and the thoracic vertebrae. The lungs themselves lack muscle tissue. The muscles responsible for respiration include the diaphragm and 11 pairs each of internal and external intercostal muscles. Intercostal muscles are muscles located between the ribs. The diaphragm forms the floor of the thoracic cavity, originating broadly between the xiphoid process (the lower tip of the breast bone), ribs 10 to 12, costal cartilage for ribs 5 to 9, and the lumbar vertebrae. It is innervated by the phrenic nerve. The diaphragm's central tendon connects by fascicles—or connective tissue—to its surrounding margins. Shaped like a dome and positioned between the abdominal and thoracic cavities, the diaphragm surrounds passageways for the esophagus and major blood vessels. The diaphragm flattens somewhat as it contracts, which increases the volume of the thoracic cage. This change forms a partial vacuum that triggers inspiration, the drawing of air into lungs.
The external intercostal muscles extend downward and behind each rib, connecting to the rib below. When the diaphragm contracts, the external intercostals draw the other ribs up and out. This inflates the lungs by expanding the thoracic cavity. Once the muscles release, the thoracic cage returns to its relaxed size, expelling air. This exhalation is smooth and relatively passive, requiring muscles only to maintain partial tension. Forced exhalation of air, as in blowing out a flame, is driven by the internal intercostal muscles. These muscles lie deep to the external intercostal muscles and also extend between the ribs, running in the opposite direction as the external intercostal muscles. Some of the muscles of the back also aid in respiration. The serratus posterior inferior and the serratus posterior superior help control rib movements during breathing. In the trunk of the body, four pairs of muscles line and support the anterior abdominal wall: the rectus abdominis muscles, the internal and external abdominal oblique muscles, and the transversus abdominis. All are innervated by intercostal nerves, and, in addition, the oblique muscles are innervated by the iliohypogastric and ilioinguinal nerves. These sheet-like muscles protect viscera—the abdominal organs—and they stabilize, flex, and rotate the vertebral column. They are important to heavy lifting and play a role in a variety of functions, including defecation, childbirth, urination, respiration, and vomiting. The rectus abdominis attaches vertically from the pubis bone to the xiphoid process and costal cartilages of ribs 5 to 7. It flexes the waist, stabilizes the pelvis, and increases internal abdominal pressure. Fibrous structures, the tendinous intersections, separate the rectus abdominis into four sections that are visible in a lean, muscled person. A fibrous rectus sheath encases the paired muscles, and a vertical line, the linea alba, marks the separation between right and left muscles. External abdominal oblique muscles run closest to the abdomen's exterior. The internal abdominal oblique muscles run deep to the external abdominal oblique muscles. Even deeper is the transversus abdominis. All insert at the xiphoid process, a small, elongated protrusion at the bottom of the sternum. It lies at the level of the T10 (thoracic 10) vertebra and is cartilaginous in young people, but usually ossified in adults age 40 and over. The fascicles of the three muscle pairs run in different directions on each muscle, together forming a strong abdominal wall. The oblique muscles assist in flexion and rotation of the vertebral column. The transversus abdominis contributes to increasing abdominal pressure and flexion.

Back muscles that act on the trunk rotate and extend the vertebral column and flex it laterally. They are categorized into deep, intermediate, and superficial groups. Muscles in the deep group connect vertebrae together; the erector spinae muscle is the prime mover in extension of the spine. It enables standing upright. The erector spinae is further divided into three complex vertical muscles: the iliocostalis, the longissimus, and the spinalis. These include cervical, thoracic, and lumbar portions and produce movements ranging from the head to the lower back. Erector spinae muscles attach across a variety of structures associated with ribs and vertebrae. The primary deep back muscle in the thoracic region is the semispinalis, which extends and bends the trunk laterally. It is divided into the semispinalis cervicis and semispinalis thoracis. The primary deep lumbar muscle is the quadratus lumborum. The quadratus lumborum and erector spinae are enclosed together in a fibrous sheath known as the thoracolumbar fascia. The multifidus muscle lies beneath this sheath. Most back muscles are innervated by the dorsal rami of the spinal nerve. The dorsal ramus is the dorsal or posterior division of the spinal nerve and it forms the dorsal nerve root after emerging from the spinal cord.

The intermediate group consists of the serratus posterior inferior and the serratus posterior superior. Rib movements during breathing are controlled by these serratus posterior muscles which lie over the erector spinae. Serratus posterior muscles attach from the spines of cervical and thoracic vertebrae to the ribs. The superficial back muscles are the trapezius, which elevates, depresses, and retracts the scapulae; the latissimus dorsi, which extends the arms and rotates them medially; the levator scapulae, which elevate the scapulae; and the rhomboideus major and minor, which retract, elevate, and rotate the scapulae.

The pelvic floor is a wall of muscle spanning the space formed by the pelvic bone. It consists of three layers of muscles combined with a sheet-like connective tissue. The pelvic floor layer just under the skin includes the superficial muscles of the perineum, which is the area between the anus and the scrotum in men, or the vulva in women. Some of these include the gluteus maximus and the superficial transverse perineal. The middle layer comprises muscles forming the urogenital triangle (for example, the ischiocavernosus and bulbospongiosus) and the anal triangle (consisting of the external anal sphincter and anal canal). Muscles making up the pelvic diaphragm form the deepest layer of the pelvic floor. This includes the levator ani, consisting of the pubococcygeus and the iliococcygeus. Most pelvic floor muscles are innervated by the pudendal nerve, which carries both sensation from the external genitalia as well as motor supply to the pelvic muscles and external urethral sphincter in both men and women.

The urethra, anal canal, and vagina pass through the pelvic floor. They open into the diamond-shaped perineum, which consists of the urogenital triangle anteriorly and anal triangle posteriorly. The perineum lies between bony structures on all sides: the pubic symphysis in front; coccyx, or tailbone, behind; and an ischial tuberosity on each side. Three muscles make up the superficial perineal space. The superficial transverse perineus attaches from the ischial tuberosities to the perineum's central tendon, supporting the pelvic floor. Paired ischiocavernosus muscles originate at the ischial tuberosities and angle to meet at the penis or clitoris, where they compress to initiate erection. The bulbospongiosus muscle originates at the perineal central tendon and plays different roles in males and females. In males it compresses the urethra, which expels semen during ejaculation. It females it contracts the vagina during intercourse. In both sexes the bulbospongiosus can be contracted to void, or expel, any remaining urine from the canal of the urethra. It exerts its action at the end of the urination process. In the middle layer or compartment of the pelvic floor, two muscles and a fibrous membrane make up the thin urogenital diaphragm that lies across the urogenital triangle. The deep transverse perineal muscle anchors at the central tendon. It supports the pelvic floor and is responsible for the expulsion of the last drops of urine from the urethra in both men and women as well as the expulsion of semen in men. The external urethral sphincter can be voluntarily constricted to prevent urination. Also in the middle pelvic compartment, the external anal sphincter muscle is in the anal triangle. Originating at the coccyx, it voluntarily constricts the anal opening to prevent defecation. The deepest pelvic layer, the pelvic diaphragm, contains two pairs of muscles. The levator ani supports pelvic organs and elevates the anus during defecation. It attaches from the pubic bone in the front of the pelvis and extends across to the sacrum and coccyx, or tailbone. The coccygeal muscle supports and lifts the pelvic floor. It originates at the ischial spine (a pointed projection of bone just inferior to the acetabulum, or hip bone) and inserts at the coccyx.