Abdominal Muscles

The abdominal muscles are important accessory muscles of forced expiration. Although quiet expiration is normally a passive process produced largely by elastic recoil of the lungs and thoracic wall, active expiration requires muscular contraction. The muscles of the anterolateral abdominal wall contribute by compressing the abdominal contents, increasing intra-abdominal pressure, and displacing the diaphragm superiorly.

The principal abdominal muscles involved in active expiration are the rectus abdominis, external oblique, internal oblique, and transversus abdominis. Their coordinated contraction also helps depress the lower ribs and stabilize the abdominal wall during forceful respiratory activity.

Role in Respiration

During quiet breathing, the abdominal muscles generally make only a limited active contribution to expiration. When ventilation must increase, however, expiration becomes active and contraction of the abdominal wall becomes increasingly important.

This occurs during activities such as exercise, coughing, sneezing, forceful speech, singing, and voluntary forced expiration.

Abdominal Muscles Involved in Expiration

The principal muscles are:

• Rectus abdominis
• External oblique
• Internal oblique
• Transversus abdominis

These muscles form much of the anterior and lateral walls of the abdomen and act together to increase pressure within the abdominal cavity.

Mechanism of Forced Expiration

Contraction of the abdominal muscles compresses the abdominal viscera and raises intra-abdominal pressure.

The abdominal contents are displaced against the inferior surface of the diaphragm, forcing the relaxed diaphragm superiorly into the thoracic cavity. This decreases the vertical dimension of the thorax and contributes to a reduction in thoracic volume.

Effect on the Diaphragm

The diaphragm is the principal muscle of inspiration, but its position is strongly influenced by abdominal pressure.

During forced expiration, abdominal muscle contraction pushes the abdominal viscera superiorly against the diaphragm. The domes of the diaphragm therefore ascend, helping expel air from the lungs.

Effect on Thoracic Volume

Superior displacement of the diaphragm decreases the vertical dimension of the thoracic cavity.

At the same time, some abdominal muscles can contribute to depression and stabilization of the lower ribs. Together these actions decrease thoracic volume and increase intrathoracic pressure during active expiration.

Rectus Abdominis

The rectus abdominis is a paired vertical muscle located on either side of the anterior midline of the abdominal wall.

It extends between the pubis inferiorly and the xiphoid process and costal cartilages of the upper abdominal region superiorly.

Respiratory Action of Rectus Abdominis

During forced expiration, rectus abdominis compresses the abdominal contents and contributes to increased intra-abdominal pressure.

Because of its superior attachments to the costal margin, it can also assist in drawing the lower thoracic cage inferiorly when the pelvis is stabilized.

Rectus Abdominis Attachments

Feature	Rectus Abdominis
Inferior attachment	Pubic symphysis and pubic crest
Superior attachment	Xiphoid process and costal cartilages of ribs 5 to 7
Major actions	Flexes trunk, compresses abdominal viscera, assists forced expiration

External Oblique

The external oblique is the most superficial of the three flat muscles of the anterolateral abdominal wall.

Its fibers generally pass inferomedially from their attachments to the lower ribs toward the abdominal aponeurosis, linea alba, iliac crest, and pubic region.

Respiratory Action of External Oblique

Contraction of the external oblique compresses the abdominal contents and increases intra-abdominal pressure.

Its attachments to the lower ribs also allow it to assist in depressing the thoracic cage during forceful expiration.

External Oblique Attachments

The external oblique arises from the external surfaces of the lower eight ribs.

Its fibers contribute to a broad aponeurosis and attach to structures including the linea alba, pubic region, and anterior portion of the iliac crest.

Internal Oblique

The internal oblique lies deep to the external oblique and superficial to transversus abdominis.

Its fibers have varying orientations, with many passing superomedially from the thoracolumbar fascia, iliac crest, and inguinal region toward the lower ribs and abdominal aponeurosis.

Respiratory Action of Internal Oblique

The internal oblique compresses and supports the abdominal viscera and contributes to increased intra-abdominal pressure during forced expiration.

Its attachments to the inferior ribs also allow it to participate in depression and stabilization of the lower thoracic cage.

Transversus Abdominis

The transversus abdominis is the deepest of the three flat anterolateral abdominal muscles.

Most of its fibers run approximately horizontally around the abdominal cavity, making the muscle particularly effective at compressing the abdominal contents.

Respiratory Action of Transversus Abdominis

Transversus abdominis plays an important role in increasing intra-abdominal pressure during active expiration.

Its contraction tightens the abdominal wall and compresses the viscera, producing superior pressure against the diaphragm.

Transversus Abdominis Attachments

The muscle arises from structures including the internal surfaces of the lower costal cartilages, thoracolumbar fascia, iliac crest, and lateral part of the inguinal ligament.

Its fibers contribute to an aponeurosis that blends with the abdominal wall structures toward the midline.

Comparison of the Expiratory Abdominal Muscles

Muscle	Fiber Orientation	Respiratory Contribution
Rectus abdominis	Predominantly vertical	Compresses abdomen and assists depression of lower thoracic cage
External oblique	Inferomedial	Compresses abdomen and assists depression of lower ribs
Internal oblique	Variable, predominantly superomedial	Compresses abdomen and stabilizes lower ribs
Transversus abdominis	Predominantly transverse	Strongly compresses abdominal contents

Quiet Expiration

Normal quiet expiration generally does not require active contraction of the abdominal muscles.

When inspiratory muscles relax, elastic recoil of the lungs and thoracic structures reduces thoracic volume and drives air outward.

Forced Expiration

During forced expiration, passive elastic recoil is supplemented by active muscular contraction.

The abdominal muscles and internal intercostal components involved in rib depression become important when air must be expelled rapidly or against increased resistance.

Sequence During Active Expiration

1. The abdominal muscles contract.
2. The abdominal wall tightens.
3. Intra-abdominal pressure increases.
4. Abdominal viscera are displaced superiorly against the diaphragm.
5. The diaphragm moves superiorly.
6. The lower ribs may be depressed or stabilized.
7. Thoracic volume decreases.
8. Intrathoracic and alveolar pressures increase relative to atmospheric pressure.
9. Air flows out of the lungs.

Relationship with the Internal Intercostal Muscles

Abdominal muscles do not act alone during vigorous expiration.

Interosseous portions of the internal intercostal muscles can depress the ribs, while the abdominal muscles increase abdominal pressure and displace the diaphragm superiorly. These mechanisms cooperate to reduce thoracic dimensions.

Lower Rib Depression

Several abdominal muscles attach directly or indirectly to the lower ribs and costal cartilages.

When their inferior attachments are stabilized, contraction can exert an inferior pull on the lower thoracic cage, assisting active expiration.

Abdominal Pressure

Increasing intra-abdominal pressure is one of the most important respiratory functions of the abdominal muscles.

The abdominal cavity contains predominantly incompressible viscera and fluid, so contraction of the surrounding muscular wall efficiently transmits pressure toward the diaphragm.

Abdominal Wall and Diaphragmatic Coupling

The diaphragm and abdominal wall function as mechanically coupled structures.

During inspiration, diaphragmatic descent displaces abdominal contents and can move the abdominal wall outward. During active expiration, abdominal contraction reverses this relationship by pushing the viscera and diaphragm superiorly.

Role During Exercise

As exercise intensity increases, ventilation rises and expiration may become increasingly active.

Recruitment of the abdominal muscles helps increase expiratory airflow and can assist in returning the diaphragm toward a favorable position for the subsequent inspiration.

Role in Coughing

The abdominal muscles are powerful contributors to the cough mechanism.

After a deep inspiration, forceful contraction of expiratory muscles generates high intrathoracic pressure. When airflow is released through the airway, rapid expiration helps dislodge and expel material from the respiratory tract.

Role in Sneezing

Sneezing also involves forceful expiratory muscle activity.

Abdominal contraction contributes to the rapid increase in pressure required to produce high-velocity expiratory airflow.

Role in Speech

Speech requires controlled expiration rather than simple passive emptying of the lungs.

Abdominal muscle activity can help regulate expiratory pressure and airflow, particularly during prolonged or forceful vocalization.

Role in Singing

Singing requires precise regulation of subglottic pressure and expiratory airflow.

Coordinated activity of the diaphragm, abdominal muscles, and thoracic muscles helps control the rate at which air leaves the lungs.

Other Functions of the Abdominal Muscles

The abdominal muscles have functions extending well beyond respiration.

They support and protect abdominal viscera, move and stabilize the trunk, contribute to posture, and generate increased intra-abdominal pressure during several physiological activities.

Valsalva-Type Actions

Strong contraction of the abdominal wall can contribute to marked increases in intra-abdominal and intrathoracic pressure when coordinated with respiratory and laryngeal mechanisms.

Such pressure generation is used during activities requiring forceful stabilization or expulsion.

Innervation

The abdominal wall muscles receive segmental innervation predominantly from the anterior rami of lower thoracic spinal nerves, with additional contribution from the first lumbar nerve to portions of the lower abdominal wall.

This segmental innervation allows coordinated activation of different regions of the abdominal musculature.

Thoracoabdominal Nerves

The anterior rami of approximately T7 to T11 continue from the intercostal spaces into the abdominal wall as thoracoabdominal nerves.

They provide motor innervation to much of the anterolateral abdominal musculature and sensory innervation to corresponding regions of the abdominal wall.

Subcostal Nerve

The anterior ramus of T12 forms the subcostal nerve.

It travels inferior to the twelfth rib and contributes motor innervation to abdominal wall muscles.

First Lumbar Nerve

Branches derived from the first lumbar nerve, particularly the iliohypogastric and ilioinguinal nerves, contribute to innervation of the inferior portions of the internal oblique and transversus abdominis.

Innervation Summary

Muscle	Principal Innervation
Rectus abdominis	Thoracoabdominal nerves, approximately T6 or T7 to T12
External oblique	Thoracoabdominal nerves and subcostal nerve
Internal oblique	Thoracoabdominal nerves, subcostal nerve, and L1 contributions
Transversus abdominis	Thoracoabdominal nerves, subcostal nerve, and L1 contributions

Abdominal Muscle Paralysis

Weakness or paralysis of the abdominal musculature can impair the ability to generate high expiratory pressures.

Quiet breathing may remain possible because the diaphragm and elastic recoil continue to function, but coughing and forceful expiration can become less effective.

Spinal Cord Injury

Neurological injury affecting the thoracic spinal pathways supplying the abdominal muscles can reduce expiratory muscle strength.

Impaired abdominal contraction can reduce cough effectiveness and make clearance of airway secretions more difficult even when diaphragmatic function is preserved.

Neuromuscular Disorders

Diseases that weaken skeletal muscles can affect both inspiratory and expiratory muscle groups.

Weak abdominal muscles may reduce maximal expiratory pressure and cough strength, contributing to impaired airway clearance.

Abdominal Surgery

Pain and mechanical limitation after abdominal surgery can reduce voluntary contraction of the abdominal wall.

This can make coughing uncomfortable or less effective and may contribute to reduced clearance of respiratory secretions during recovery.

Assessment of Expiratory Muscle Function

Respiratory muscle function can be evaluated using measurements that reflect the ability to generate expiratory pressure and airflow.

Reduced expiratory pressure may reflect weakness of the abdominal muscles, other expiratory muscles, or broader neuromuscular dysfunction.

Relationship to Forced Vital Capacity

During a forced vital capacity maneuver, the subject inspires deeply and then expires as forcefully and completely as possible.

Abdominal muscle recruitment contributes to the active expiratory effort, particularly as lung volume decreases and continued forceful expiration is required.

Key Anatomical Relationships

• The abdominal muscles are accessory muscles of active expiration.
• Quiet expiration is predominantly passive.
• Rectus abdominis, external oblique, internal oblique, and transversus abdominis participate in forced expiration.
• Contraction compresses the abdominal viscera.
• Compression increases intra-abdominal pressure.
• Increased abdominal pressure displaces the diaphragm superiorly.
• Superior diaphragmatic movement decreases thoracic volume.
• Several abdominal muscles also assist in depressing or stabilizing the lower ribs.
• Abdominal muscles cooperate with other active expiratory muscles.
• They are strongly recruited during coughing and vigorous exercise.
• They contribute to controlled expiratory airflow during speech and singing.
• Weakness can impair forceful expiration and effective coughing.

Clinical Importance

The abdominal muscles provide an important mechanism for active expiration by linking the abdominal and thoracic compartments mechanically. Their contraction increases intra-abdominal pressure and pushes the diaphragm superiorly, reducing thoracic volume and assisting the generation of expiratory airflow.

This function becomes particularly important when expiratory demands exceed those of quiet breathing. Coughing, exercise, forceful respiratory maneuvers, speech, and other activities can require coordinated recruitment of the abdominal wall.

Understanding the abdominal muscles as expiratory muscles provides the anatomical foundation for studying forced expiration, diaphragmatic mechanics, intra-abdominal pressure, cough physiology, respiratory muscle weakness, thoracoabdominal innervation, and the mechanics of ventilation.