A group of lung conditions characterized by an impaired ability of the lungs to expand. Causes include disorders that affect the lung parenchyma (e.g., interstitial lung disease, pneumonia, pulmonary edema), pleura, and pleural cavity, deformities of the thorax, and respiratory muscle weakness. Typically causes ↓ vital capacity, ↓ total lung capacity, normal/↓ FEV1 (due to ↓ in vital capacity), and a normal or ↑ FEV1:FVC ratio.