Which muscle is the primary inspiratory muscle?

• A. Diaphragm
• B. Intercostal muscles
• C. Abdominal muscles
• D. Pectoralis major

Answer: (A)

In breathing, the diaphragm is the primary muscle that drives inhalation. When the diaphragm contracts, it moves downward and flattens, enlarging the vertical dimension of the thoracic cavity. This expansion lowers intrathoracic pressure and pulls air into the lungs during quiet breathing. Other muscles, like the intercostals, assist by expanding the chest, and abdominal muscles are mainly involved in forced expiration, or can aid deep inspiration only when breathing is very labored. The pectoralis major can help with deep, strenuous inspiration by lifting the chest, but it isn’t the main muscle used for normal inspiration.