What causes fatigue during different types of exercise, and how does the body recover afterwards?
Explain the causes of fatigue specific to each energy system and the recovery processes that restore the body, including the oxygen deficit and EPOC.
The causes of fatigue specific to each energy system, the oxygen deficit and EPOC, replenishment of PC and glycogen, lactate removal and active versus passive recovery.
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What this dot point is asking
You must explain why fatigue occurs for each energy system, and describe how the body recovers, including the oxygen deficit, EPOC, and the restoration of fuels and clearance of by-products.
What causes fatigue
Fatigue is multi-causal, but the dominant cause matches the dominant energy system.
- ATP-PC system: fatigue is caused by depletion of phosphocreatine stores, which are very small. Performance drops within about 10 seconds of maximal effort.
- Anaerobic glycolysis: fatigue is associated with the accumulation of hydrogen ions, which lower muscle pH (acidosis) and interfere with muscle contraction and glycolytic enzymes. Lactate is a marker of this, not the direct cause.
- Aerobic system: fatigue over long durations is caused by glycogen depletion, dehydration, a rise in core temperature, and electrolyte loss.
Recovery processes
Recovery restores the body to its pre-exercise state.
- EPOC (excess post-exercise oxygen consumption) is the elevated oxygen uptake after exercise that repays the oxygen deficit. The fast component restores ATP and phosphocreatine and re-oxygenates myoglobin and blood; the slow component supports the removal of by-products, restoration of body temperature and hormone levels.
- Phosphecreatine restoration is rapid: about half restored in 30 seconds and almost fully within 2 to 3 minutes.
- Glycogen restoration is slow: it can take 24 to 48 hours and depends heavily on carbohydrate intake.
- Lactate and hydrogen ion removal occurs as lactate is oxidised for fuel, converted back to glucose in the liver, or excreted. Light activity speeds this clearance.
- Rehydration and electrolyte replacement restore blood volume and muscle function.
Active versus passive recovery
- Active recovery is light, continuous exercise (a slow jog or swim) after a hard effort. It keeps blood flow elevated, speeding lactate and hydrogen ion clearance and PC restoration. It generally clears lactate faster than rest.
- Passive recovery is complete rest. It is preferred when the priority is restoring phosphecreatine (short, maximal efforts) or when the athlete is exhausted.
Implications for training and performance
Understanding recovery lets you plan work-to-rest ratios. Repeated sprint training needs nearly full PC recovery between efforts (long rests), while building lactate tolerance deliberately uses incomplete recovery. Endurance performance depends on carbohydrate loading and rehydration because glycogen and fluid restoration are slow.