[lee-usmle]

This is frequently tested in USMLE

1. First few seconds the muscle utilizes what ATP store available.
2. Next few seconds the muscle utilizes Creatine Phosphate.
3. After 10 seconds Glycogen would have provided the necessary glucose to kick off glycolysis (that's why fast twitch white muscles have more glycogen).
4. Glycolysis will result in lactic acid.
5. Lactic acid will convert to pyruvate which will enter the Krebs cycle for oxidative phosphorylation.
6. For each glucose molecule coming from glycogen there will be 36 ATPs and glycogen will be depleted quickly, but it will replenish later on when the exercise stops.
7. If exercise continues then the source of glucose will be gluconeogensis from the liver.
8. If exercise continues further then ATP will come from fatty acid oxidation (that's why slow twitch red muscle are rich in fatty acids).