Which muscle group plays a significant role in expiration?

• A. External intercostals
• B. Diaphragm
• C. Scalenes
• D. Abdominals

Answer: (D)

The abdominal muscles play a significant role in expiration, particularly during forced breathing or physical exertion. During expiration, especially when it is active, the abdominal muscles contract to increase intra-abdominal pressure. This contraction helps push the diaphragm upwards, thereby reducing the volume of the thoracic cavity and facilitating the expulsion of air from the lungs.

While the diaphragm is primarily responsible for quiet inhalation and works to lower the thoracic cavity during inspiration, and the external intercostals contribute to the expansion of the thoracic cavity during inhalation, the function of expiration—especially during active breathing—leans heavily on the abdominal muscles. The scalenes, located in the neck, assist with inhalation rather than expiration. Thus, the primary contribution of the abdominal muscles during the process of expiration underscores their importance in the mechanics of ventilation.