Fatigue during exercise can result from phosphocreatine depletion, glycogen depletion, and acidosis. Their relative importance depends mainly on exercise intensity and duration, and fatigue usually reflects several interacting processes rather than one cause.
How each factor causes fatigue
| Factor | When it is important | Mechanism causing fatigue |
|---|---|---|
| Phosphocreatine (PCr) depletion | Short-duration, maximal or near-maximal exercise, such as sprinting or weightlifting | PCr rapidly donates a phosphate group to ADP to resynthesize ATP. As intramuscular PCr stores fall, the rate of ATP resynthesis through the phosphagen system decreases, so the muscle cannot maintain maximal power output. |
| Glycogen depletion | Prolonged endurance exercise and repeated high-intensity efforts | Muscle glycogen supplies glucose for glycolysis and aerobic respiration. When stores become low, carbohydrate oxidation decreases and greater reliance is placed on slower fat oxidation, reducing the rate at which ATP can be supplied. Low liver glycogen may also reduce blood glucose, contributing to central fatigue. |
| Acidosis | Sustained high-intensity exercise with rapid anaerobic glycolysis and ATP turnover | Accumulation of hydrogen ions lowers muscle pH. This can interfere with enzyme activity, calcium handling and actin-myosin cross-bridge function, reducing force production. |
PCr depletion develops within seconds, whereas severe glycogen depletion generally develops over a much longer period. After exercise, PCr is restored relatively quickly through oxygen-dependent aerobic metabolism, while glycogen restoration requires carbohydrate intake and may take many hours.
A common misconception is that lactic acid directly causes fatigue. Lactate is not simply a waste product and can be oxidized as a fuel; the fatigue-related mechanism is more accurately linked to hydrogen-ion accumulation and reduced pH, alongside other changes within the muscle.
Exam technique
For A.3.3 Fatigue and recovery (HL), distinguish the mechanisms rather than merely listing them. Link PCr depletion to reduced rapid ATP resynthesis, glycogen depletion to reduced carbohydrate availability, and acidosis to impaired muscle function. Always relate each cause to the intensity and duration of exercise.