During expiration the chest wall muscles relax, narrowing the space around the lungs and expelling air. Diseases that cause weakness of the muscles of the chest wall and diaphragm include muscular dystrophy and myasthenia gravis. The resulting paralysis is similar to the deliberate paralysis caused by curare-like agents that are used in anesthesia. Shortness of breath occurs in diseases that inflame the nerves that control the muscles. These conditions are called neuropathy and include the Guillain Barre Syndrome.
Shortness of breath occurs in diseases of the spinal cord.
The spinal cord controls the muscles including the muscles of breathing. Spinal cord diseases include Lou Gehrig's disease and polio. During inspiration they do not expand fully. And during expiration they do not relax fully. Thus, the bellows function of the lungs is compromised.