Duchenne Muscular dystrophy (DMD) is an inherited disorder, which usually affects boys (it is very rare in girls). The muscles become weaker as boys get older. This is because the body cannot make the muscle protein called dystrophin. This makes the muscle cells weak and they gradually break down. Signs of weakness start when the boys are between 3 and 5 years of age (sometimes earlier). At first, the weakness is seen mostly in the legs and hips. The children may:
Eventually this weakness also makes walking more difficult and a wheelchair is needed. Gradually, all the muscles become very weak, including the muscles used for breathing and the heart.
Information taken from http://muscle.ca/discover-md/types-of-neuromuscular-disorders/