The phosphagen, glycolytic, and oxidative energy systems are three pathways that resynthesize adenosine triphosphate (ATP), the molecule that directly supplies energy for muscular contraction. All three operate simultaneously, but their relative contributions depend mainly on exercise intensity and duration.
How the systems resynthesize ATP
Muscles store only enough ATP for approximately one to two seconds of maximal activity. The energy systems therefore continually add a phosphate group to ADP to resynthesize ATP.
| Energy system | Mechanism and fuel | Main exercise role |
|---|---|---|
| Phosphagen system | Uses stored ATP and phosphocreatine in the sarcoplasm. Phosphocreatine rapidly donates a phosphate to ADP without requiring oxygen. | Provides ATP at the highest rate but has the lowest capacity. It dominates during explosive activity lasting up to about ten seconds, such as a sprint start or maximal lift. |
| Glycolytic system | Uses glucose or muscle glycogen through anaerobic glycolysis in the sarcoplasm. ATP is produced by substrate-level phosphorylation, with lactate accumulating when glycolytic activity is high. | Provides ATP rapidly during high-intensity activity lasting roughly ten seconds to two minutes, such as a 400 m sprint. |
| Oxidative system | Uses carbohydrates and fats, and occasionally protein, through aerobic reactions in the mitochondria. Most ATP is produced by oxidative phosphorylation. | Produces ATP more slowly but has the greatest capacity. It predominates during prolonged, lower-intensity exercise and recovery. |
These systems form an energy continuum, not an on-off sequence. For example, the oxidative system contributes from the beginning of a sprint, although the phosphagen system initially supplies ATP at the greatest rate.
A common misconception is that each system begins only when the previous one stops. In reality, exercise intensity, duration, training status, oxygen availability, and fuel availability alter each system's relative contribution.
Exam technique
For an IB SEHS comparison, identify each system's fuel, oxygen requirement, cellular location, rate and capacity of ATP production, and dominant exercise context. Avoid giving duration boundaries as absolute cut-offs; explain that dominance changes along a continuum.
