Final answer:
The muscles of inspiration include the diaphragm and external intercostal muscles, which help to expand the thoracic cavity and draw air into the lungs. For expiration, the diaphragm and intercostals relax for passive breathing, but the internal intercostals and abdominal muscles facilitate forced expiration by compressing the thoracic cavity.
Step-by-step explanation:
The muscles involved in inspiration and expiration are crucial to the process of pulmonary ventilation, in which air enters and leaves the lungs. The muscles of inspiration include the diaphragm and external intercostal muscles. The diaphragm contracts and flattens, enlarging the thoracic cavity and reducing the pressure inside, allowing air to flow into the lungs. The external intercostal muscles help by elevating the ribs, thus expanding the rib cage. For expiration, which is generally a passive process during normal breathing, the diaphragm and external intercostals relax, causing the lungs to recoil and air to be expelled. However, during forced expiration, such as during strenuous activity, the internal intercostal muscles and abdominal muscles are activated to expel air more aggressively by drawing the ribs together and compressing the abdominal cavity, respectively.