What muscle group is primarily responsible for breathing by expanding the chest cavity?

The intercostal muscles play a crucial role in the process of breathing, particularly in expanding and contracting the chest cavity. These muscles are located between the ribs and are divided into external and internal intercostal muscles. When the external intercostal muscles contract, they lift the rib cage and increase the volume of the thoracic cavity. This expansion lowers the pressure within the cavity compared to the atmospheric pressure, allowing air to flow into the lungs during inhalation.

In contrast, the abdominal muscles primarily assist with forced exhalation rather than the initial expansion of the chest cavity. Chest muscles, often referring to the pectoral muscles, do not have a primary role in breathing mechanics. The deltoids, which are shoulder muscles, do not contribute to the breathing process at all. Therefore, the intercostal muscles are specifically designed and positioned to facilitate the primary breathing action of expanding the chest cavity.