Fibrodysplasia Ossificans Progressiva

FOP affects mobility because the body's joints, such as the knees or the elbows, connect the bones and aid in movement. In FOP, extra bone replaces the ligaments (which cover the joints), as well as muscles and tendons (which move the joints). Consequently, movement in areas affected by FOP becomes difficult or impossible.

When the extra bone is grown, the muscles that constantly move are suddenly stationary and can't move. If your arm was suddenly encased in a cast all the way to the shoulder it would be hard to do anything. But Imagine that wih the pain of having the cast be growing inside of you for 6 to 8 weeks at a time nonstop. It makes living normaly impossible. ALS slowy restricts the patient's brain ability to make muscles work and eventually they will be frozen.