Explain the causes of fatigue specific to each energy system and the recovery processes that restore the body, including the oxygen deficit and EPOC.

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.

The recovery rates also explain why the same session can train different qualities depending on the rest interval. Because phosphocreatine restores in two to three minutes but glycogen takes a day or more, a coach manipulates rest to target the desired adaptation: long rests preserve power and develop the ATP-PC system, while short, incomplete rests force reliance on anaerobic glycolysis and build buffering capacity and lactate tolerance. Matching the work-to-rest ratio to the energy system being developed, and timing recovery nutrition to refill glycogen, is the applied reasoning that links this dot point to program design and the Performance Improvement task.