Final answer:
The diaphragm is the main muscle that contracts during inspiration to keep the airway open while awake. The external intercostal muscles also help by moving the ribs to increase thoracic volume, and during forced breathing.
Step-by-step explanation:
The muscle that contracts during inspiration to keep the airway open while awake is the diaphragm. When the diaphragm contracts, it moves inferiorly and increases the volume of the thoracic cavity.
This is accompanied by the contraction of the external intercostal muscles, which move the ribs upward and outward, further increasing the thoracic cavity volume. This expansion of the thoracic cavity is a critical aspect of inspiration, establishing a pressure gradient that allows air to flow into the lungs.
Additionally, during forced breathing or hyperpnea, which may occur during activities like exercise or singing, other accessory muscles, such as the scalenes in the neck, also contract to lift the thoracic wall, substantially increasing lung volume.
In summary, the diaphragm is the main muscle responsible for inspiration while awake, with intercostal muscle support. During forced breathing, additional muscle groups are recruited to further facilitate airway opening and lung ventilation.