During expiration, what happens to the diaphragm and intercostal muscles?

• A. They relax
• B. They contract
• C. They contract then relax
• D. They remain contracted

Answer: (A)

During quiet expiration, breathing is passive. The diaphragm relaxes and moves upward from its flattened, contracted position, and the intercostal muscles relax as the chest wall returns toward its resting shape. This relaxation allows the lungs to recoil elastically, decreasing the thoracic volume and increasing the pressure inside the airways, which pushes air out. Active contraction of these muscles would instead raise the chest cavity and promote inspiration, so for normal expiration they relax.