A cough is a powerful, protective reflex designed to keep the respiratory airways clear of irritants, foreign particles, and excess mucus. This rapid, involuntary sequence relies on the coordinated contraction and relaxation of numerous muscle groups throughout the chest and abdomen. This muscular effort generates explosive forces, effectively clearing the airways. The process is a three-stage action that transforms a deep breath into a high-velocity expulsion of air.
Phase One: Drawing Air In
The initial stage requires a deep, preparatory inhalation, forcefully expanding the chest cavity to maximize lung volume. The primary muscle for this inspiratory phase is the diaphragm, a large, dome-shaped muscle beneath the lungs. When the diaphragm contracts, it flattens and moves downward, significantly increasing the vertical volume of the thoracic cavity.
The external intercostal muscles, located between the ribs, supplement this action. They contract to pull the rib cage upward and outward, further expanding the chest and decreasing internal pressure. Accessory muscles of inspiration are also recruited for the deep, rapid breath needed for an effective cough. Muscles like the scalenes and the sternocleidomastoid in the neck lift the upper ribs and clavicles, contributing to the large intake of air.
Phase Two: Building Internal Pressure
Once the deep breath is taken, the muscular action shifts to the compression phase, sealing the air within the chest and rapidly building pressure. This stage begins with the tight closure of the glottis, the opening between the vocal cords, which seals the airway. Laryngeal adductor muscles within the larynx contract forcefully to maintain this seal.
Simultaneously, a powerful contraction of the expiratory muscles begins, pushing against the trapped air. The abdominal wall muscles—including the rectus abdominis, transversus abdominis, and the oblique muscles—tighten and compress the abdominal contents. This action pushes the diaphragm upward, decreasing the chest cavity volume while the glottis remains closed. The internal intercostal muscles also contract, pulling the rib cage downward and inward. This opposing muscular action drastically increases the intra-thoracic pressure, which can reach up to 300 millimeters of mercury (mmHg).
Phase Three: The Rapid Expulsion
The final stage is the explosive expulsion phase, which clears the airways. This phase is initiated by the sudden opening of the tightly sealed glottis and vocal cords. The air, compressed to high pressure, is suddenly released, generating an explosive airflow.
This explosive force is powered by the continued, massive contraction of the core expiratory muscles. The abdominal muscles—the rectus abdominis and the internal and external obliques—contract with maximum force. They act like a piston to compress the chest cavity and push the remaining air out. The internal intercostal muscles also continue contracting to depress the ribs, further reducing the thoracic volume. This coordinated muscular power creates airflow velocities that can approach or exceed 800 kilometers per hour, dislodging mucus and foreign matter.