Most of the glycogen metabolism disorders that affect skeletal muscle involve enzymes of glycogenolysis (myophosphorylase ( PYGM ), glycogen debranching enzyme ( AGL ), phosphorylase b -kinase ( PHKB )) and glycolysis (phosphofructokinase ( PFK ), phosphoglyceromutase ( PGAM 2), aldolase A ( ALDOA ), β -enolase ( ENO 3)); however, 3 of them involve glycogen synthesis (glycogenin-1 ( GYG 1), glycogen synthase ( GSE ), and debranching enzyme ( GBE 1)). Many present with exercise-induced cramps and rhabdomyolysis with more intense exercise (ie, PYGM , PFK , PGAM 2), while others present with muscle wasting and weakness ( GYG 1, AGL , GBE 1). Failure of serum lactate to rise with exercise, with an exaggerated response to ammonia, is a common but not invariant feature. Serum creatine kinase ( CK ) levels are often elevated in myopathic forms and PYGM deficiency , but may be normal and elevated only in rhabdomyolysis ( PGAM 2, PFK , ENO 3). Therapy for glycogen storage diseases that result in exercise-induced symptoms involves lifestyle adaptations and carefully selected exercises. Immediate carbohydrate ingestion before exercise improves symptoms in glycogenolytic defects (i.e., PYGM),
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