Final answer:
The diaphragm and external intercostal muscles contract during deep breathing to allow the lungs to fill with air.
Explanation:
The diaphragm is the primary muscle that contracts during deep breathing to allow more room for the lungs to fill with air. Upon contraction, the diaphragm moves inferiorly towards the abdominal cavity, expanding the thoracic cavity and creating space for the lungs to fill with air. Additionally, the external intercostal muscles contract to expand the rib cage, further increasing thoracic volume for air intake.
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