Fatigue mechanisms: WACE Physical Education Studies Unit 3
“Explain the causes of fatigue in different intensities and durations of exercise and relate them to the energy systems”
Fatigue is the decline in performance during exercise, and its cause depends on intensity and duration. Short maximal efforts fatigue from phosphocreatine depletion. High intensity efforts fatigue from hydrogen ion accumulation that raises muscle acidity. Long endurance efforts fatigue from glycogen depletion, dehydration and rising body temperature. Matching the cause to the type of effort is the key skill.
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.
It is the build up of hydrogen ions and the resulting fall in muscle pH that impairs contraction, rather than lactate being directly harmful. Lactate is actually a usable fuel. When explaining high intensity fatigue, point to the hydrogen ions raising acidity and disrupting enzymes and contraction, which is the precise mechanism examiners look for, instead of vaguely blaming lactic acid.
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.
Matching cause to event
A marker wants intensity and duration linked to the fatigue cause. Model answer: A 100 metre sprint is a maximal effort of about ten seconds, powered mainly by the ATP-PC system, so fatigue is caused by the depletion of phosphocreatine stores rather than lactate. A marathon is a long submaximal effort powered mainly by the aerobic system, so fatigue is caused by glycogen depletion as carbohydrate stores run low, together with dehydration reducing blood volume and rising body temperature. The cause of fatigue differs because the predominant energy system and the duration differ.
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.
Students apply lactic acid to every type of fatigue. In a long submaximal endurance event, the athlete is working aerobically and lactate is not accumulating, so the fatigue is from glycogen depletion, dehydration and rising temperature, not acidity. Reserve hydrogen ion and acidity explanations for high intensity efforts in the anaerobic glycolytic zone, and use fuel depletion and heat for endurance.
Exam-style questions
Questions in the style of SCSA exam questions on this dot point, each with a worked answer. They are written by ExamExplained unless tagged "Past paper"; the year shows the paper a question is modelled on.
WACE 2022-style6 marksCompare the main causes of fatigue in a 400 m sprint with those in a marathon, relating each cause to the predominant energy system in that event.Show worked answer →
A 6 mark compare answer needs the contrasting fatigue causes tied to the energy system of each event.
- 400 m sprint (anaerobic glycolysis)
- Fatigue is mainly caused by the rapid accumulation of hydrogen ions (acidosis) as glucose is broken down without oxygen. The fall in muscle pH impairs glycolytic enzymes and cross-bridge function, forcing the sprinter to slow late in the race. Phosphocreatine depletion also contributes early on.
- Marathon (aerobic system)
- Fatigue is mainly caused by glycogen depletion (hitting the wall), along with dehydration, raised core temperature and reduced blood glucose over the long duration. Acidosis is not the main factor because intensity is submaximal and by-products are removed.
- Conclusion
- Short maximal events fatigue chiefly through metabolic by-product accumulation; long endurance events fatigue chiefly through fuel depletion and thermoregulatory/fluid factors.
Markers reward hydrogen-ion accumulation for the sprint, glycogen depletion (plus dehydration/temperature) for the marathon, and the link to each predominant energy system.
WACE 2024-style3 marksExplain why hydrogen ion accumulation causes fatigue during high-intensity anaerobic exercise.Show worked answer →
A 3 mark explain answer needs the cause-and-effect chain.
- Source
- During high-intensity work, anaerobic glycolysis breaks down glucose without oxygen, producing hydrogen ions faster than they can be buffered or removed.
- Effect on muscle
- The build-up lowers muscle pH (acidosis), which inhibits the enzymes controlling glycolysis and interferes with calcium binding and cross-bridge cycling.
- Result
- Force production falls and the athlete must reduce intensity, so performance declines, which is experienced as fatigue.
Markers reward hydrogen-ion build-up lowering pH, impaired enzyme/cross-bridge function, and the resulting drop in force output.