186 Anatomy & Physiology Essentials Copyright Goodheart-Willcox Co., Inc. Collectively, the trunk muscles enable flexion, extension, hyperextension, lateral flexion, and rotation of the head and trunk. From a functional perspective, the anterior abdominal muscles also assist with urination, defecation, forced expiration during breathing, and childbirth. The all-important diaphragm muscle regulates breathing. The trunk muscles also serve as a protective sheath for the organs of the thoracic and abdominal cavities. The locations and primary functions of the major muscles of the anterior and posterior trunk are summarized in the table in Figure 6.16 and are shown in Figure 6.17. Anterior Trunk Muscles Running the entire length of the anterior abdomen, the superficial, paired rectus abdominis muscles are flat and encased in a connective tissue called the rectus sheath. The muscles are separated by a longitudinal band of connective tissue called the linea alba. This, in combination with several lateral bands of connective tissue, divides the muscle into eight muscle bellies, strengthen their neck and trunk muscles in an effort to maximize spinal stability and minimize risk of injury to the delicate spinal cord and internal organs. Conversely, gymnasts train to enhance the flexibility of the spine and are capable of extraordinary spinal hyperextension, especially during balance-beam and floor-exercise routines (Figure 6.15). Muscles of the Trunk Anterior Muscles Muscle Origin Insertion Primary Functions Rectus abdominis pubic crest sternum and ribs 5–7 flexion and lateral flexion of trunk External oblique ribs 5–12 anterior iliac crest and pubis flexion, lateral flexion, and rotation to opposite side of trunk Internal oblique anterior iliac crest ribs 10–12 flexion, lateral flexion, and rotation to same side of trunk Posterior Muscles Muscle Origin Insertion Primary Functions Trapezius occipital bone, nuchal ligament, and vertebrae C7–T3 clavicle and scapular spine extension and hyperextension of head elevation, depression, and adduction of scapula Erector spinae: iliocostalis ilium and ribs ribs and transverse processes of vertebrae extension, lateral flexion, and rotation to opposite side Erector spinae: longissimus inferior aspect of transverse processes of vertebrae superior aspect of transverse processes of vertebrae extension, lateral flexion, and rotation to opposite side Erector spinae: spinalis inferior aspect of spinous processes superior aspect of spinous processes extension, lateral flexion, and rotation to opposite side Quadratus lumborum iliac crests upper lumbar vertebrae individually assist lateral flexion together assist trunk extension Muscles for Breathing Muscle Origin Insertion Primary Functions Diaphragm (separates thoracic and abdominal cavities) enlarges thoracic cavity for inhalation Internal intercostals (between ribs) decrease thoracic cavity volume during forced expiration External intercostals (between ribs) help enlarge thoracic cavity volume during inhalation Figure 6.16 Goodheart-Willcox Publisher Jiang Dao Hua/Shutterstock.com Figure 6.15 Female gymnasts train to increase flexibility, especially in the lumbar spine.