GLOSSARY

Muscle glycogen is the storage form of glucose in muscle: a branched polysaccharide stored as granules in the sarcoplasm and the main fuel for moderate to intense activity.

Characteristics

• Storage amount: about 300–500 grams in all the muscles of a trained adult; the liver stores about 80–100 grams.
• Local use: muscle glycogen is used only by that muscle, because muscle lacks the enzyme glucose-6-phosphatase and cannot release glucose into the blood, unlike the liver, which regulates blood sugar.
• Water: each gram of glycogen is stored with about 3 grams of water.

Relevance to Sport

• Fatigue in endurance: glycogen depletion is associated with severe fatigue, which marathon runners call “hitting the wall”, usually around kilometre 30.
• Carbohydrate loading: consuming about 8–12 grams of carbohydrate per kilogram of body weight per day for 36–48 hours before events longer than 90 minutes increases stores.
• During activity: 30–90 grams of carbohydrate per hour in prolonged activities.
• Post-exercise resynthesis: fastest in the first hours after exercise; about 1–1.2 grams per kilogram per hour for rapid recovery between closely spaced sessions.
• Training with low glycogen: some endurance athletes train with low stores in a periodised way to enhance mitochondrial adaptations; this should not be used before quality sessions or competition.