Glycogen storage disease type V (GSD V) or McArdle disease is an inherited disorder of glycogen metabolism that affects the skeletal muscle. The disease is characterized by muscle fatigue, stiffness, myalgia, and weakness often caused by activity and improved by rest. If activity is continued after symptoms appear, severe, painful muscle cramping and contracture may occur; this may be accompanied by myoglobinuria and can lead to rhabdomyolysis with possible renal failure if not treated properly. A subset of patients have fixed muscle weakness and wasting with age. Serum creatine kinase (CK) is generally elevated. Reports suggest that statin use may cause an adverse muscle reaction in these patients or uncover the underlying muscle disease. Onset of symptoms is usually in early childhood; however, most people are not diagnosed until after age 30 years. In one U.S. study, the incidence of GSD V was estimated to be about 1 in 100,000.