Which muscle is primarily responsible for breathing?

The diaphragm is primarily responsible for breathing. This dome-shaped sheet of skeletal muscle forms the floor of the chest and separates the thoracic cavity from the abdomen.

When the diaphragm contracts, it flattens and moves downward. That increases the volume of the thoracic cavity, lowers pressure inside it, and draws air into the lungs. When it relaxes during quiet breathing, it returns toward its domed position and helps exhalation.

The intercostal muscles between the ribs also assist respiration, especially by moving the rib cage. During forceful breathing, additional muscles in the neck, chest and abdomen contribute. They are important helpers, but the diaphragm is the main driver of ordinary inhalation.

A common mix-up is saying that the lungs themselves are muscles. They are elastic organs, not muscles; the diaphragm and other respiratory muscles change chest volume so the lungs expand and recoil. The phrenic nerves, arising mainly from spinal cord levels C3–C5, control the diaphragm.