Which muscle helps with breathing out?

The internal intercostals play a vital role in forced expiration, which is the active process of expelling air from the lungs. During this process, the internal intercostals contract, which pulls the ribs downward and inward, reducing the volume of the thoracic cavity. This action increases the pressure in the lungs as the diaphragm relaxes, helping to push air out more forcefully.

In contrast, the external intercostals and diaphragm primarily assist with inhalation. The diaphragm is the main muscle responsible for breathing in; it contracts and moves downward to create negative pressure that draws air into the lungs. The sternocleidomastoid, while involved in neck movement and accessory breathing during strenuous activities, does not play a prominent role in the normal process of expiration. Thus, the internal intercostals are the correct choice for the muscle that aids in the process of breathing out.