IN 1951, McArdle1 described a patient whose major symptom was limitation of vigorous activity because of cramps after exercise. When electromyographic examination was performed during a cramp, electrical silence was found despite maximum shortening of the affected muscle; this could be considered a contracture (shortening of muscle without propagated action potential2). McArdle recognized the similarity of this event to the classical experiments of Lundsgaard3 in which muscle poisoned by iodoacetate was able to contract without formation of lactic acid. When the patient exercised forearm muscles under ischemic conditions, in contrast to a normal person, there was no rise in venous lactic acid. McArdle therefore postulated that the disorder was due to a defeat of muscle glycogen breakdown.
The nature of the enzymatic abnormality was clearly established by Schmid and his colleagues in one family,4 and by Pearson, Mommaerts, and their colleagues in another.5 Inability