What is the main function of the external intercostals?

The primary function of the external intercostals is to facilitate quiet inspiration, which is the process of breathing in air at rest without any extra exertion. During quiet inspiration, the external intercostal muscles contract, leading to an increase in the volume of the thoracic cavity. This increase creates a negative pressure that allows air to flow into the lungs. The external intercostals help elevate the ribs during this process, contributing to the expansion of the chest wall and making it easier for air to enter the lungs.

In contrast, forced expiration involves the contraction of other muscles, such as the internal intercostals and abdominal muscles, which work to actively push air out of the lungs. Quiet expiration generally occurs passively when the diaphragm relaxes after contraction during inspiration. Forceful inspiration, while it involves additional muscles like the sternocleidomastoid and scalenes, is not the primary role of the external intercostals but rather an extension of what they can assist with when the body demands more air intake than during normal, quiet breathing.