Phosphocreatine for seconds, glycolysis for minutes, oxidative for hours
Mnemonic
Three systems, in order of how quickly they can supply ATP:
- Phosphocreatine (ATP-PC) - immediate, lasts about 10 seconds. Anaerobic, no lactate. Sprinting
- Anaerobic glycolysis - seconds to about 2 minutes. Produces lactate. 400 metres
- Oxidative phosphorylation - minutes to hours. Requires oxygen, uses carbohydrate then fat. Endurance
“Fast and brief, or slow and sustained”: the systems trade power for capacity, and all three operate at once with the balance shifting over time.
Yields make the point: anaerobic glycolysis gives 2 ATP per glucose, oxidative metabolism about 30 to 32.
Fibre types follow the same division: type I slow oxidative fibres are red, fatigue resistant and packed with mitochondria; type IIx fast glycolytic fibres are pale, powerful and fatigue quickly; type IIa is intermediate.
Expansion
| System | Duration | Fuel | Yield |
|---|---|---|---|
| ATP-phosphocreatine | 0 to 10 seconds | Stored ATP and phosphocreatine | Immediate, tiny |
| Anaerobic glycolysis | 10 seconds to about 2 minutes | Glucose and glycogen | 2 ATP per glucose, fast |
| Oxidative phosphorylation | Beyond 2 minutes | Carbohydrate, fat, protein | About 30 ATP per glucose, slow |
Stored ATP alone lasts only 1 to 2 seconds, which is why phosphocreatine exists as an immediate buffer, regenerated by creatine kinase.
Anaerobic glycolysis produces lactate. Lactate itself is not the cause of fatigue or of delayed muscle soreness; it is a useful fuel shuttled to the heart and liver, and the associated hydrogen ions account for the acidosis.
Fat provides the most ATP per gram but requires oxygen and is slow to mobilise, so it dominates in prolonged low-intensity exercise, while carbohydrate dominates at higher intensities. This crossover underlies the practice of carbohydrate loading and the phenomenon of “hitting the wall” when muscle glycogen is exhausted.