ALS is characterized by muscle weakness and atrophy—muscle wasting. It is due to the degeneration of upper and lower motor neurons of the body that are responsible for voluntary movement. Affected individuals ultimately lose their ability to initiate and control all voluntary movement, though bowel and bladder function, as well as eye movements, are typically spared. While behavioral and cognitive functioning usually remains intact, a small percentage may show signs of frontotemporal dementia. The disease can strike at any age but most commonly develops between the ages of 55 and 75.