It refers to inward movement of the lower rib cage during inspiration,[2] - instead of outward as is normal - implying a flat, but functioning, diaphragm, often associated with COPD.[3] COPD, and more specifically emphysema, often lead to hyperexpansion of the lungs due to air trapping. The resulting flattened diaphragm contracts inwards instead of downwards, thereby paradoxically pulling the inferior ribs inwards with its movement.