A marathon runner begins a race at a steady pace. After the first few kilometres, the runner's breathing rate and heart rate increase significantly. Towards the end of the race, the runner's leg muscles begin to feel painful and fatigued. Explain why the runner's muscles become painful towards the end of the race, and describe what must happen in the body after the race finishes to allow the runner to recover.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Marathon runners typically complete a race of 42 kilometres. During the later stages of a marathon, many runners experience muscle pain and fatigue in their legs.
Model answer (5 marks)
During the later stages of a marathon the muscles cannot receive enough oxygen to satisfy their energy demand, so they switch to anaerobic respiration.
1. Oxygen supply falls behind demand.
2. Anaerobic glycolysis is used.
3. Glucose is broken down to lactic acid.
4. Lactic acid accumulates, lowering pH and causing pain and fatigue.
5. After the race the body must breathe deeply to take in extra oxygen, which oxidises the lactic acid and repays the oxygen debt.
1. Oxygen supply falls behind demand.
2. Anaerobic glycolysis is used.
3. Glucose is broken down to lactic acid.
4. Lactic acid accumulates, lowering pH and causing pain and fatigue.
5. After the race the body must breathe deeply to take in extra oxygen, which oxidises the lactic acid and repays the oxygen debt.
Examiner tips
- Use the exact terms ‘anaerobic respiration’, ‘lactic acid’, and ‘oxygen debt’.
- Show the sequence: low O₂ → anaerobic glycolysis → lactic acid build‑up → pain/fatigue → post‑race oxygen debt repayment.
- Keep the answer concise – one sentence per point.
- Mention the need for extra oxygen after the race to oxidise lactic acid.
Common mistakes
- Confusing aerobic with anaerobic respiration; forgetting to mention oxygen debt.
- Using vague phrases like ‘muscle fatigue’ without linking to lactic acid.
- Leaving out the post‑race breathing/oxygen debt repayment step.
Mark scheme (5 marks)
- Towards the end of the race the muscles cannot get enough oxygen to meet energy demands
- So the muscles respire anaerobically (without oxygen)
- Glucose is broken down to lactic acid (during anaerobic respiration)
- The build-up of lactic acid causes the pain/fatigue in the muscles
- After the race, the runner must continue to breathe deeply/take in extra oxygen to oxidise/break down the lactic acid — this is the oxygen debt (repaying the oxygen debt)
Related
- All Pearson Edexcel A-Level Biology A (9BI0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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