A certain strain of mutant mice is characterized by limited ability to engage in prolonged exercise. after a high carbohydrate meal, one of these mice can exercise on a treadmill for only about 30% of the time a normal mice can. at exhaustion, blood glucose levels in the mutant mouse are quite low, and they increase only marginally after rest. when liver glycogen in fed mutant mice is examined before exercise, the polymers have chains that are highly branched, with average branch lengths of about 10 glucose residues in either alpha 1,4 or alpha 1,6 linkage. glycogen from exhausted normal mice has the same type of structure. glycogen from exhausted mutant mice is still highly branched, but the polymer has an unusually large number of single glucose residues with alpha 1,6 linkages. practically all the chains with alpha 1,4 linkages are still about 10 residues. explain the metabolic and molecular observations concerning the mutant mice