Physiology
The three energy systems
All three systems run at once. What changes with exercise intensity and duration is which one dominates.
Side by side
| System | Fuel | Duration | Power | Oxygen? |
|---|---|---|---|---|
| Phosphagen (ATP-PC) | Stored ATP, creatine phosphate | ~0–10 s | Highest | No |
| Glycolytic (anaerobic) | Glucose, muscle glycogen | ~10 s–2 min | High | No |
| Oxidative (aerobic) | Carbohydrate, fat, some protein | 2 min onward | Lowest | Yes |
The trade-off is consistent: the faster a system can produce ATP, the shorter it can sustain that output.
How each works
- Phosphagen — creatine phosphate donates a phosphate to ADP, regenerating ATP almost instantly. This is the system creatine supplementation targets, and it powers a maximal sprint or a heavy set.
- Glycolytic — glycolysis breaks glucose down to pyruvate, which becomes lactate when demand outpaces oxygen delivery. Fast, but limited.
- Oxidative — pyruvate and fatty acids enter the Krebs cycle and electron transport chain. Far slower to ramp up, but its capacity is enormous.
Lactate is not the cause of soreness
Lactate clears within roughly an hour of exercise, whereas delayed-onset muscle soreness peaks 24–48 hours later. DOMS is associated with mechanical damage and inflammation, not lingering lactate. Lactate is also a usable fuel — the heart and liver take it up readily.
Why fat can't fuel sprinting
Fat carries more energy per gram than carbohydrate, but oxidizing it requires more oxygen per unit of ATP and proceeds through more steps. It cannot deliver ATP fast enough for high-intensity effort. This is the physiological reason carbohydrate availability limits high-intensity performance, and why carb loading matters for endurance events.
Frequently asked questions
What are the three energy systems?
The phosphagen (ATP-PC) system for efforts up to about 10 seconds, the glycolytic system for roughly 10 seconds to 2 minutes, and the oxidative (aerobic) system for sustained effort beyond about 2 minutes.
Does lactic acid cause muscle soreness?
No. Lactate clears within about an hour of exercise, while delayed-onset muscle soreness peaks 24 to 48 hours later. DOMS is associated with mechanical damage and inflammation rather than lactate.
Why can't fat fuel high-intensity exercise?
Oxidising fat requires more oxygen per unit of ATP and involves more metabolic steps than using carbohydrate, so it cannot supply ATP quickly enough to sustain high-intensity effort.
Keep reading: Glycolysis explained · Carb loading for athletes · Creatine explained