Energy Systems and Metabolism
Domain 2 - Energy Systems and Metabolism
This domain tests how well you match intensity and duration to ATP supply, and whether you can interpret lactate, respiratory exchange ratio (RER), and post-exercise oxygen consumption without oversimplifying.
Core rules and concepts
- ATP–phosphocreatine system: immediate, maximal power for roughly 0–10 seconds; limited by phosphocreatine stores.
- Anaerobic glycolysis: roughly 10 seconds to 2 minutes; glucose breakdown produces lactate, which is a fuel and signaling molecule, not the cause of delayed-onset muscle soreness.
- Oxidative phosphorylation: dominant beyond about 2 minutes and at lower intensities; uses carbohydrate and fat, with mitochondrial capacity and oxygen delivery as key limits.
- RER near 0.7 suggests predominant fat use; near 1.0 suggests carbohydrate use; above 1.0 usually reflects hyperventilation or non-steady-state conditions.
- Lactate threshold is the intensity where lactate appearance exceeds clearance; it is trainable and predicts endurance performance.
- EPOC is elevated oxygen uptake after exercise; its calorie cost is modest and not a major weight-loss strategy.
Worked example
A 60-second maximal effort: ATP–PC dominates the first seconds, glycolysis supplies most of the remainder, and oxidative metabolism contributes a small but real portion. A 5 km race is predominantly oxidative, with a glycolytic surge at the finish. Reasoning: duration and intensity determine the blend of systems, not an on/off switch.
Traps that cost marks
- Claiming lactate causes soreness; microdamage and inflammation are better explanations.
- Calling the “fat-burning zone” the best zone for fat loss; total energy balance matters more.
- Overstating EPOC as a large calorie burner.
Exam takeaway
Name the dominant system, then explain the contribution of the others and the limiting factor.