At rest, the aerobic energy system dominates. During high-intensity exercise, the dominant system depends mainly on duration: the ATP-PC system dominates very short, explosive activity, while anaerobic glycolysis becomes dominant as maximal activity continues.
How Energy-System Dominance Changes
All three energy systems operate simultaneously. However, their relative contributions change according to exercise intensity, duration, fuel availability, and the athlete's training status.
| Situation | Dominant energy system | Main explanation |
|---|---|---|
| Rest | Aerobic system | Oxygen supply meets the low ATP demand; fats provide much of the fuel, alongside carbohydrates. |
| Very high intensity for about 0-10 seconds | ATP-PC system | Stored ATP and phosphocreatine rapidly resynthesize ATP without requiring oxygen. |
| High intensity for roughly 10 seconds to 2 minutes | Anaerobic glycolysis | Glucose or glycogen is broken down rapidly without directly using oxygen, producing ATP and lactate. |
| Prolonged exercise at lower intensity | Aerobic system | Carbohydrates and fats are oxidized in the mitochondria to provide ATP sustainably. |
The ATP-PC system has the highest rate of ATP production but a very limited capacity because phosphocreatine stores are rapidly depleted. It is therefore important in sprint starts, jumps, throws, and maximal lifts.
As intense exercise continues, anaerobic glycolysis makes a greater contribution. It produces ATP less rapidly than the ATP-PC system but has a larger capacity. During recovery, the aerobic system supports phosphocreatine restoration, lactate metabolism, and the return to resting conditions.
A common misconception is that only one energy system operates at a time. In reality, energy-system contribution exists on a continuum; “dominant” means that one system supplies the greatest proportion of ATP at that moment.
Exam Technique
For IB SEHS subtopic A.2.3 Energy systems, identify the dominant system and justify it using both intensity and duration. Avoid stating simply that “high intensity is anaerobic”: distinguish short explosive activity from longer high-intensity activity, and acknowledge that all systems contribute.
