This is frequently tested in USMLE
- First few seconds the muscle utilizes what ATP store available.
- Next few seconds the muscle utilizes Creatine Phosphate.
- After 10 seconds Glycogen would have provided the necessary glucose to kick off glycolysis (that's why fast twitch white muscles have more glycogen).
- Glycolysis will result in lactic acid.
- Lactic acid will convert to pyruvate which will enter the Krebs cycle for oxidative phosphorylation.
- 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.
- If exercise continues then the source of glucose will be gluconeogensis from the liver.
- If exercise continues further then ATP will come from fatty acid oxidation (that's why slow twitch red muscle are rich in fatty acids).