rare hereditary deficiency of the enzyme glycogen phosphorylase in muscle cells. In the absence of this enzyme, muscles cannot break down animal starch (glycogen) to meet the energy requirements of exercise. Muscle activity is thus solely dependent on the availability of glucose (blood sugar) and other nutrients in the circulating blood. Victims of McArdle's disease are chronically weak because their muscles are incapable of prolonged exertion; even moderate exercise produces muscle cramping and severe pain. Unlike most other types of glycogenosis, the disease is not fatal, and the missing enzyme does not impair the functioning of other body systems. McArdle's disease is inherited as an autosomal recessive trait.