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McArdle disease (glycogen storage disease type V, OMIM database number 232600) may provide the ultimate model of exercise intolerance in humans, and thus is of great interest in the sports medicine setting. The condition is an autosomal recessive disorder of muscle glycogen metabolism originally described in 1951 by Brian McArdle.1 Patients have pathogenic mutations in both alleles of the PYGM gene, which encodes myophosphorylase, the skeletal muscle isoform of glycogen phosphorylase.2 As a result, myophosphorylase activity is totally absent. Because this enzyme initiates the breakdown of muscle glycogen leading to liberation of glucose-1-phosphate, patients are unable to obtain energy from their muscle glycogen stores. Hence this disease is arguably the paradigm of exercise intolerance in humans.2 Exercise is the trigger for symptom occurrence in McArdle patients; as such, they tend to be averse to exercise and have often been advised by clinicians to refrain from exercise.
McArdle disease and exercise
Men and women are equally affected and symptoms usually begin during childhood, typically in the school playground or physical education classes.3 The clinical presentation is dominated by exercise intolerance in the form of acute crises of early fatigue and contractures, which are often accompanied by exertional rhabdomyolysis, as indicated by marked increases in serum levels of creatine kinase (CK) activity and myoglobinuria.3 Rhabdomyolysis can sometimes be so severe as to cause acute renal failure. A typical feature of the disease is the so-called ‘second wind’ phenomenon, that is, marked improvement in the tolerance of aerobic, dynamic, large muscle mass exercise (walking …