AbstractA hallmark of glycogen storage disease type II, caused by defective α‐glucosidase (AGLU) activity, is a progressive decline in muscle performance. The objective of this study was to determine the relative contribution to this decline in muscle performance of (1) decline in muscle mass; (2) decline in muscle protein content per unit mass; and (3) accumulation of glycogen. To this end, isometric torque and power in AGLU−/− mice at 7, 13, and 20 months were assessed in situ. Power (∼24 mW) and torque (∼2.45 Nmm) did not change with age in control animals, but declined significantly in AGLU−/− mice, in the three age groups. No decline in protein content per unit muscle mass was observed. Muscle atrophy explained one third of the decline in muscle performance; the remaining part was attributed to a decrease in muscle quality—a decrease in mechanical performance per unit muscle mass. Mechanical effects of glycogen inclusions could not fully explain this decrease. Additional factors must therefore play a role. Muscle Nerve, 2005