Spinal muscular atrophy (SMA) is a term for a group of inherited neuromuscular diseases. All forms of the disease affect specialized nerve cells called lower motor neurons in the base of the brain and the spinal cord. These which control the movement of voluntary muscles, used for activities such as crawling, walking, head and neck control and swallowing. Most affected are the proximal muscles, or in other words the muscles closest to the point of origin, in this case those closest to the trunk of one's body. Weakness in the legs is generally greater than weakness in the arms.
1 in 40 people are carriers of the gene for SMA; 1 in every 6,000 live births are affected.