Which of the following muscles is primarily engaged during forced inspiration?

• A. Sternocleidomastoids
• B. Diaphragm
• C. Internal intercostals
• D. External intercostal muscles

Answer: (D)

The external intercostal muscles are primarily engaged during forced inspiration because their primary function is to elevate the ribs and expand the thoracic cavity. When these muscles contract, they increase the volume of the chest cavity, allowing for a greater inflow of air into the lungs. This is particularly important during activities that require increased respiratory effort, such as exercise or labored breathing.

In contrast, while the diaphragm plays a crucial role in normal inspiration by flattening to increase the thoracic volume, it is not the muscle that primarily engages during forced inspiration since it is mostly activated in quiet breathing. The sternocleidomastoids assist in elevating the sternum and are accessory muscles that come into play when more intensive breathing efforts are required. Internal intercostals primarily contribute to forced expiration rather than inspiration. Thus, the external intercostal muscles are the most relevant in facilitating an increase in airflow during forced inspiration.