Which muscle is the primary muscle of inhalation, providing the major expansion of the thoracic cavity during quiet breathing?

• A. diaphragm
• B. internal intercostals
• C. external intercostals
• D. abdominal wall

Answer: (A)

In quiet inhalation, the main driver is the diaphragm. This dome-shaped muscle sits at the bottom of the thoracic cavity; when it contracts, it moves downward and flattens, increasing the vertical dimension of the chest. That expansion lowers the intrathoracic pressure, allowing air to flow into the lungs. The external intercostals assist by lifting the ribs to widen the rib cage, but their contribution is secondary to the diaphragm’s bulk movement. Internal intercostals and abdominal wall muscles are more involved in forced expiration, not the basic inhalation during resting breathing. So the diaphragm is the primary muscle responsible for the major expansion of the thoracic cavity in quiet breathing.