What is the primary function of the external intercostal muscles?

• A. To compress the abdominal cavity
• B. To elevate the ribs and expand the chest cavity
• C. To assist the diaphragm in exhalation
• D. To stabilize the thoracic spine

Answer: (B)

The primary function of the external intercostal muscles is to elevate the ribs and expand the chest cavity. These muscles are located between the ribs and play a crucial role during inhalation. When the external intercostal muscles contract, they pull the ribs upward and outward, which increases the volume of the thoracic cavity. This expansion creates a negative pressure within the chest, allowing air to flow into the lungs. This process is essential for efficient breathing, especially during periods of increased respiratory demand, such as exercise.

In contrast, other options describe functions that are not primarily associated with the external intercostal muscles. For instance, compressing the abdominal cavity relates more to other muscle groups like the abdominal muscles, and assisting the diaphragm is more about the role of the internal intercostals during forced expiration instead of the external intercostals, which primarily assist during inhalation. Stabilizing the thoracic spine is also not the main function of the external intercostals; thoracic stability involves other muscle systems in the body. Understanding the role of external intercostals is fundamental in the study of respiratory mechanics.