Explain the causes of fatigue in different intensities and durations of exercise and relate them to the energy systems

What this dot point is asking

WACE expects you to identify the main causes of fatigue and to select the dominant cause for a given effort based on its intensity and duration. Linking the cause back to the relevant energy system shows full understanding.

What fatigue is

Fatigue is the inability to maintain the required power output or intensity, a reversible decline in performance during exercise. It is not a single thing; different efforts fatigue for different reasons, which is why the cause must be matched to the type of activity.

Fatigue in short maximal efforts

In all out efforts of a few seconds, the limiting factor is the depletion of phosphocreatine, the fuel of the ATP-PC system. Once phosphocreatine stores fall, the very fast resupply of ATP cannot be maintained and power drops. There is no significant lactate involved here; the fatigue is about running low on the immediate energy store, which is why a series of maximal sprints needs adequate rest to restore phosphocreatine.

Fatigue in high intensity efforts

In efforts from about ten seconds to two minutes, the anaerobic glycolytic system dominates and produces lactic acid, which dissociates into lactate and hydrogen ions. The accumulation of hydrogen ions raises the acidity in the muscle. This increased acidity interferes with the enzymes of glycolysis and with the contraction process, reducing the muscle's ability to produce force. This is the burning sensation and sudden power loss in events like a 400 metre sprint.

Fatigue in long endurance efforts

In prolonged submaximal exercise the aerobic system dominates and the causes of fatigue change. Glycogen depletion is central: as muscle and liver carbohydrate stores run low, the athlete cannot sustain the pace and must slow or rely more on fat, which supplies energy more slowly. Dehydration reduces blood volume and impairs the delivery of oxygen and the removal of heat. Rising core body temperature (hyperthermia) further stresses the body and impairs performance. These combine to cause the gradual slowing seen late in a marathon.

How this maps to the exam

A question often names an event or describes a decline in performance and asks for the cause of fatigue. First fix the intensity and duration, then choose the dominant cause: phosphocreatine depletion for short maximal work, hydrogen ion accumulation for high intensity work, and glycogen depletion, dehydration and heat for endurance work. Link it to the predominant energy system.