The external intercostal muscles are attached between adjacent ribs and oriented such that their contraction elevates the anterior end of each rib, pulling it upward towards a horizontal plane. This action increases the antero-posterior diameter of the rib or thoracic cage. From side view, it resembles the action of a "pump handle."

While the importance of external intercostal muscles is overshadowed by the diaphragm, their contraction prevents the rib cage from being pulled downward and inward as the diaphragm descends during inspiration. While the external intercostals can maintain a considerable level of breathing on their own, their paralysis does not prevent ventilation by the diaphragm alone.