Energy Systems and Metabolism
Energy Systems and Metabolism
This domain tests the ATP-PCr, glycolytic, and oxidative systems, substrate use, oxygen uptake, lactate threshold, and metabolic adaptations to training. You need to link fuel needs to intensity and duration rather than memorizing isolated definitions.
Core rules
- ATP-PCr: immediate, maximal efforts of roughly 0–10 seconds; recovery is fast but incomplete with short rest.
- Glycolysis: fast but finite; dominates high-intensity efforts from about 10 seconds to 2 minutes.
- Oxidative phosphorylation: dominant for sustained work beyond about 2 minutes; uses carbohydrate and fat.
- Respiratory exchange ratio (RER): near 0.70 suggests mostly fat oxidation; near 1.00 suggests mostly carbohydrate oxidation. RER is not a direct measure of “fat burning” in all contexts.
- Lactate is a fuel and signaling molecule, not simply a waste product; lactate accumulation is associated with acidosis, but H+ production is more complex.
- EPOC is the excess post-exercise oxygen consumption; it is real but usually modest compared with exercise energy expenditure.
Worked example
A 400 m sprinter finishes in 55 seconds. The race is dominated by glycolysis, with some ATP-PCr at the start and oxidative contribution at the end. Carbohydrate availability is therefore critical; a high-carbohydrate meal 3–4 hours before and adequate daily carbohydrate support the session. A marathoner at 4-minute/km pace relies mainly on oxidative metabolism and benefits from carbohydrate loading and during-race carbohydrate. Reasoning: match fueling strategy to the dominant system and duration.
Traps
Believing a low-intensity “fat-burning zone” maximizes total fat loss; blaming lactate alone for muscle burn; overestimating EPOC; and assuming fat oxidation is always preferable for performance.