Final answer:
The diaphragm and external intercostal muscles contract during normal quiet inspiration, enabling air to enter the lungs. They relax during normal quiet expiration, allowing air to exit. Forced breathing involves additional muscles, such as the scalenes and abdominal muscles, to aid in the process.
Step-by-step explanation:
During normal quiet inspiration, the primary muscles involved are the diaphragm and the external intercostal muscles. As the diaphragm contracts, it moves downward, increasing the volume of the thoracic cavity.
The external intercostal muscles help in this process by lifting the ribs upward and outward, further expanding the thoracic cavity and decreasing intra-alveolar pressure, which allows air to be drawn into the lungs due to the pressure gradient created.
In contrast, during normal quiet expiration, these muscles relax; the diaphragm moves upward, and the elastic recoil of the lungs, together with the relaxation of the intercostals, reduce the thoracic volume, which increases the intra-alveolar pressure and drives air out of the lungs.
For forced breathing or hyperpnea, such as during exercise, additional muscles come into play. During forced inspiration, accessory muscles including the scalenes and possibly the sternocleidomastoid muscles help to further increase the volume of the thoracic cavity.
Whereas for forced expiration, muscles like the internal intercostals and the abdominal muscles (e.g., obliques) contract to forcefully reduce the thoracic cavity volume and push more air out of the lungs.