Which statement about intercostal muscles is true?

• A. External intercostals are muscles of inhalation.
• B. Internal intercostals are used during inhalation.
• C. External intercostals are used during forced exhalation.
• D. Internal intercostals are the muscles of inhalation.

Answer: (A)

The main idea is that inhalation relies on muscles that expand the chest. External intercostal muscles run between the ribs and contract to lift the ribs up and outward, which increases the volume of the thoracic cavity and draws air into the lungs. This makes them the primary muscles used for inhalation. Internal intercostals, on the other hand, mainly act during expiration to depress the ribs (with the exception of their interchondral portions, which can assist with inspiration, but they are not the primary inhalation muscles). They are not the main muscles of inhalation, and the external intercostals are not used for forced exhalation (that role is filled by the internal intercostals and abdominal muscles). So the statement that external intercostals are muscles of inhalation is true.