Which muscle is primarily responsible for the act of breathing?

The diaphragm is primarily responsible for the act of breathing because it serves as the main muscle that facilitates the process of inhalation and exhalation. When the diaphragm contracts, it moves downward, which increases the volume of the thoracic cavity and decreases the pressure inside it. This pressure difference allows air to flow into the lungs. During exhalation, the diaphragm relaxes and moves back up, pushing air out of the lungs.

While the intercostal muscles assist by helping expand and contract the ribcage, and the abdominal muscles can play a role in forced exhalation, the diaphragm is the key muscle that initiates and drives the normal breathing process. The sternocleidomastoid muscle, primarily involved in neck mobility and assisting in deep inhalation during heavy breathing, does not play the primary role in the basic act of breathing. Thus, the diaphragm is clearly the most critical muscle for this essential bodily function.