Which of the following muscles is primarily responsible for expiration?

• A. Diaphragm
• B. Accessory inspiratory muscles
• C. External intercostals
• D. Abdominal muscles

Answer: (D)

The abdominal muscles play a crucial role in the process of expiration, particularly during forced breathing. When the abdominal muscles contract, they push the diaphragm upward by compressing the abdominal contents, effectively reducing the volume of the thoracic cavity. This action increases the pressure in the thoracic cavity, helping to expel air from the lungs more efficiently, especially during vigorous physical activity or speaking.

In contrast, the diaphragm is the primary muscle of inspiration and is most active during inhalation. Accessory inspiratory muscles assist in deep breathing but do not primarily facilitate expiration. The external intercostals primarily aid in inhalation by elevating the ribs and expanding the thoracic cavity, rather than contributing significantly to expiration. Therefore, the abdominal muscles are identified correctly as the primary muscles responsible for expiration, particularly in active breathing scenarios.