Final answer:
The diaphragm is the primary muscle that contracts during normal quiet inspiration to move air into the lungs. As it contracts, it lowers and flattens, increasing thoracic cavity volume and creating a pressure differential that draws air in. The external intercostal muscles also assist by raising and expanding the rib cage.
Step-by-step explanation:
During normal quiet inspiration, the muscle that contracts to cause air to move into the lungs is the diaphragm. This dome-shaped skeletal muscle plays a critical role in breathing. When the diaphragm contracts, it moves downward and flattens out, increasing the vertical dimensions of the thoracic cavity.
Concurrently, the external intercostal muscles between the ribs also contract, pulling the ribs upward and outward. The combined action of the diaphragm and intercostal muscle contraction increases the thoracic cavity's volume, which in turn lowers the pressure inside the lungs below that of the atmosphere. This pressure differential drives air into the lungs, allowing for inhalation.
During expiration, these muscles relax, and the elasticity of the lung tissues allows the lungs to recoil, which increases intrapulmonary pressure and forces air out in a passive process.