Glycogen : Carbohydrate fuel for exercise

Storage form of carbohydrates in muscles and liver, the main fuel above the aerobic threshold.

Glycogen is polymerised glucose stored mainly in muscles (~400-500 g) and liver (~80-100 g), representing about 1,600-2,400 kcal total : enough for ~90-120 min of hard effort. Beyond that, the 'marathon wall' or 'bonk' occurs if exogenous carbohydrate intake is insufficient. The higher the intensity, the faster glycogen depletion. In Ironman (10-12 h), in-race fuelling is critical to maintain stores.