McArdle disease (sometimes called McArdle's disease or muscle phosphorylase deficiency) is a metabolic disorder , more specifically a glycogen storage disease, caused by a deficiency of the enzyme muscle phosphorylase (sometimes known as myophosphorylase). This enzyme helps break down glycogen (a form of stored carbohydrate) into glucose so that it can be utilized within the muscle cell.

People with this disease experience difficulty when their muscles are called upon to perform relatively brief yet intense activity. The inability to break down glycogen into glucose results in an energy shortage within the muscle, resulting in muscle pain and cramping, and sometimes causing serious injury to the muscles. In addition, rhabdomyolysis—the breakdown of muscle tissue—can cause myoglobinuria , a red-to-brown-colored urine. The myoglobinuria can cause kidney damage. The disease is hereditary and is inherited as an autosomal recessive trait.