Which muscle is the primary muscle of breathing?

• A. Oblique Muscles
• B. Abdominal Muscles
• C. Intercostal Muscles
• D. Diaphragm

Answer: (D)

Breathing in quiet conditions is driven mainly by a muscle that contracts to expand the chest cavity. The diaphragm is a large, dome-shaped muscle that separates the thoracic and abdominal cavities. When it contracts, it moves downward, increasing the vertical dimension of the thoracic cavity and allowing the lungs to expand. This expansion lowers intrapleural pressure, creating a suction that pulls air into the airways. Because this muscle does most of the work to enlarge the thoracic volume, it serves as the primary driver of inspiration during normal breathing. It is innervated by the phrenic nerve (C3–C5), which explains why damage to that pathway can severely impair breathing.

The intercostal muscles assist by lifting and expanding the rib cage, augmenting ventilation during deeper or more rapid breaths, while the abdominal muscles come into play mainly during forced expiration by increasing intra-abdominal pressure and pushing the diaphragm upward. The oblique abdominal muscles are part of the abdominal wall and do not initiate inspiration.