Anatomy and Physiology 2e · The Muscular System
Axial Muscles of the Abdominal Wall, and Thorax
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In 30 seconds
The front and sides of your trunk are walled by muscles with two distinct jobs: moving the spine and compressing the abdominal contents. The anterolateral abdominal wall is built from four paired muscles — the rectus abdominis Long vertical muscle of the front of the abdomen Full entry →, external oblique Superficial lateral wall muscle, fibers run down-and-in Full entry →, internal oblique Middle lateral wall layer, fibers run up-and-in Full entry →, and transversus abdominis Deepest wall layer, horizontal fibers Full entry → — whose flat tendons (aponeuroses) meet at the midline in the linea alba Midline "white line" where wall aponeuroses meet Full entry →. Contracting together, they compress the abdomen like a pressurized cylinder, producing forced expiration, coughing, defecation, urination, and the pushing of childbirth; individually they flex, rotate, and laterally flex the trunk. Behind them, the quadratus lumborum Posterior wall muscle from iliac crest to rib 12 Full entry → anchors the lower spine and rib cage.
The thorax is walled and floored by the muscles of breathing. The diaphragm Dome-shaped muscle separating thorax and abdomen Full entry → — a dome-shaped sheet separating the thoracic and abdominal cavities — is the primary muscle of inspiration: when it contracts it flattens, enlarging the thorax and drawing air in. The external intercostals assist quiet inspiration by elevating the ribs, while the internal intercostals and abdominal muscles power forced expiration.
Why this matters
The abdominal wall is the body's "core": it stabilizes the spine, protects the abdominal organs, and generates the pressure behind coughing, sneezing, defecation, urination, and childbirth. Weakness or separation of these muscles (with aging, pregnancy, or surgery) reduces that support, and openings in the wall are hernia sites — among the most common surgical problems in the world. The respiratory muscles matter even more directly: the diaphragm drives most of every breath, and its failure — from spinal cord injury, neuromuscular disease, or fatigue — is life-threatening. This also explains why people with weak abdominal muscles cough weakly and why core and breathing training are so often prescribed in rehabilitation.
The college version
Core Concepts
The four muscles of the anterolateral abdominal wall
- Rectus abdominis — a long, vertical, parallel-fibered muscle from the pubis to the rib cage, with tendinous intersections that give the "six-pack" appearance. It flexes the trunk and compresses the abdominal contents. The paired rectus muscles are separated by the linea alba and enclosed in the rectus sheath.
- External oblique — the most superficial lateral muscle; fibers run downward and inward (like hands in pockets). One side laterally flexes and rotates the trunk to the opposite side; both sides compress.
- Internal oblique — the middle layer; fibers run upward and inward, perpendicular to the external oblique. One side rotates the trunk to the same side; both sides compress.
- Transversus abdominis — the deepest layer; fibers run horizontally, like a corset. It barely moves the trunk; it compresses and supports the abdominal contents and raises intra-abdominal pressure.
The three flat muscles end in broad aponeuroses that wrap around the rectus abdominis (the rectus sheath) and meet at the midline as the linea alba. Inferiorly, the external oblique aponeurosis Flat, sheet-like tendon Full entry → folds back as the inguinal ligament Folded lower edge of the external oblique aponeurosis Full entry → (anterior superior iliac spine to pubic tubercle) — a landmark and part of the inguinal canal, a common hernia site.
Quadratus lumborum
A thick, quadrilateral muscle of the posterior abdominal wall, from the iliac crest to the 12th rib and lumbar vertebrae. It laterally flexes the trunk, extends the lumbar spine, and fixes the 12th rib so the diaphragm has a stable anchor — a common source of low back pain, "the muscle of the low back."
The diaphragm: the primary muscle of inspiration
The diaphragm is a large, dome-shaped skeletal muscle forming the floor of the thoracic cavity and the roof of the abdominal cavity; its fibers converge on a central tendon with three major openings: the aortic hiatus (aorta), the esophageal hiatus (esophagus), and the caval opening (inferior vena cava). When it contracts, the dome flattens and descends, increasing thoracic volume; intrapleural pressure drops and air flows into the lungs. Relaxation returns it to its dome and expiration occurs passively. During deep inspiration, the scalenes and sternocleidomastoid (accessory muscles) elevate the ribs and sternum to enlarge the thorax further.
The intercostal muscles and forced expiration
Between the ribs lie three layers of intercostal muscles. The external intercostals run downward and forward; they elevate the ribs during quiet inspiration, expanding the thorax. The internal intercostals run at right angles to them (downward and backward) and depress the ribs. During forced expiration — coughing, blowing, heavy exercise — the internal intercostals contract with the abdominal muscles: the wall compresses the viscera, pushing the relaxed diaphragm up, and the internal intercostals pull the ribs down, shrinking the thorax and driving air out.
How It Works / Step-by-Step Process
A deep breath in and out: (1) Quiet inspiration: the diaphragm flattens; the external intercostals lift the ribs. Thoracic volume increases, pressure drops, air flows in. (2) Deep inspiration: the scalenes and sternocleidomastoid add force, elevating the ribs and sternum further. (3) Quiet expiration: the diaphragm and external intercostals relax; elastic recoil expels the air. (4) Forced expiration (cough): the abdominal wall compresses the abdomen, pushing the relaxed diaphragm up, while internal intercostals pull the ribs down — the thorax shrinks and air is forced out.
Common Confusions
| Do Not Confuse | With | Difference |
|---|---|---|
| External oblique rotation | Internal oblique rotation | External rotates opposite; internal rotates same side |
| Rectus abdominis | Transversus abdominis | Rectus flexes the trunk; transversus compresses and stabilizes |
| External intercostals | Internal intercostals | External elevate ribs (inspiration); internal depress ribs (forced expiration); fibers run at right angles |
| Diaphragm | Intercostals | The diaphragm is primary; intercostals assist by moving the ribs |
| Inguinal ligament | Inguinal canal | Ligament = folded aponeurosis edge; canal = the passage above it where hernias occur |
| Abdominal compression | Trunk flexion | Compression raises intra-abdominal pressure (cough/push); flexion bends the trunk — transversus only compresses |
Eli explains
The same idea, in plain words
Explain it like I’m 10
Your belly is wrapped in muscle layers that squeeze together like a stretchy belt — that's what helps you cough, sneeze, and push. Your main breathing muscle is a dome under your ribs called the diaphragm: when it flattens, your chest gets bigger and air rushes in, like opening an accordion.
Worked example
A cough, dissected. Someone coughs to clear their airway. First, a deep breath in: diaphragm and external intercostals enlarge the thorax. Then the glottis closes while the abdominal wall muscles — rectus abdominis, both obliques, and transversus abdominis — contract hard. Intra-abdominal pressure rises sharply, pushing the contents and the relaxed diaphragm upward while the internal intercostals pull the ribs down. With the glottis suddenly open, air blasts out at high speed, carrying mucus with it. Now consider why a person with weak abdominal muscles (after abdominal surgery, for example) has an ineffective cough: the pressure-generating "belt" is gone — a real clinical consequence.
Key takeaways
- Wall layers (superficial → deep): external oblique → internal oblique → transversus abdominis, plus the vertical rectus abdominis; aponeuroses meet at the linea alba.
- External oblique rotates the trunk opposite; internal oblique the same side; rectus abdominis flexes; transversus abdominis compresses/supports.
- All wall muscles together compress the abdomen → forced expiration, coughing, defecation, urination, childbirth; the breath-held Valsalva version also briefly raises blood pressure (educational note, not for casual practice).
- Inguinal ligament = folded inferior edge of the external oblique aponeurosis; related to the inguinal canal, a common hernia site.
- Quadratus lumborum: lateral flexion of the trunk; fixes rib 12 for the diaphragm.
- Diaphragm = primary inspiratory muscle; contraction flattens it → thoracic volume ↑ → air in. Three openings: aortic hiatus, esophageal hiatus, caval opening.
- External intercostals elevate the ribs (inspiration); internal intercostals depress them (forced expiration). Accessory muscles: scalenes, sternocleidomastoid.
Check yourself
6 review questions from the chapter. Try each one, then open the answer.
List the four muscles of the anterolateral abdominal wall from superficial to deep.
Show answer
External oblique, internal oblique, transversus abdominis (superficial to deep), with the rectus abdominis running vertically in the midline.
What happens to the thoracic cavity when the diaphragm contracts, and why does air enter the lungs?
Show answer
The diaphragm flattens and descends, increasing thoracic volume; intrapleural pressure drops below atmospheric and air flows in.
Which muscles produce forced expiration, and how do they work together?
Show answer
Forced expiration uses the internal intercostals (depressing the ribs) and abdominal wall muscles (pushing the relaxed diaphragm up), shrinking the thorax and driving air out.
Which abdominal muscle layer runs horizontally like a corset, and what is its main job?
Show answer
The transversus abdominis; it compresses and supports the abdominal contents and raises intra-abdominal pressure.
Where is the quadratus lumborum located, and what two actions does it perform?
Show answer
In the posterior abdominal wall, from iliac crest to rib 12 and lumbar vertebrae; it laterally flexes the trunk and fixes rib 12 during breathing.
Name the three openings of the diaphragm.
Show answer
The aortic hiatus (aorta), esophageal hiatus (esophagus), and caval opening (inferior vena cava).
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- rectus abdominis
- Long vertical muscle of the front of the abdomen
- external oblique
- Superficial lateral wall muscle, fibers run down-and-in
- internal oblique
- Middle lateral wall layer, fibers run up-and-in
- transversus abdominis
- Deepest wall layer, horizontal fibers
- aponeurosis
- Flat, sheet-like tendon
- linea alba
- Midline "white line" where wall aponeuroses meet
- inguinal ligament
- Folded lower edge of the external oblique aponeurosis
- quadratus lumborum
- Posterior wall muscle from iliac crest to rib 12
- diaphragm
- Dome-shaped muscle separating thorax and abdomen
- external / internal intercostals
- Rib muscles with fibers running down-and-forward / down-and-backward
Sources & references
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