A marathon runner completes a 26-mile race. During the final few miles, the runner's muscles begin to ache and feel tired. Explain why the runner's muscles ache towards the end of the race, and describe what happens in the runner's body after the race to remove the cause of this ache.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Marathon runners often experience muscle fatigue and aching in the later stages of a long race. This is linked to changes in the type of respiration occurring in their muscle cells.
Model answer (4 marks)
During the final miles the runner’s muscles cannot receive enough oxygen to meet the demand, creating an oxygen debt. The muscles therefore switch to anaerobic respiration, producing lactic acid as a by‑product. The accumulation of lactic acid lowers the pH of the muscle, which irritates nerve endings and causes the aching and fatigue. After the race the runner continues to breathe deeply, taking in extra oxygen; this oxygen is used to oxidise the lactic acid back to pyruvate and CO₂, removing the cause of the ache.
Examiner tips
- Use the terms ‘oxygen debt’, ‘anaerobic respiration’ and ‘lactic acid’.
- Explain the pH change and link it to pain.
- Show the recovery process – deep breathing and oxidation of lactic acid.
Common mistakes
- Confusing aerobic with anaerobic respiration.
- Saying the muscles ‘run out of oxygen’ instead of ‘cannot supply oxygen fast enough’.
- Forgetting to mention the post‑race deep breathing and oxidation of lactic acid.
Mark scheme (4 marks)
- Towards the end of the race, the runner cannot supply oxygen to muscles fast enough / there is an oxygen debt
- Anaerobic respiration occurs in the muscles, producing lactic acid
- The build-up of lactic acid causes the muscle ache / pain / fatigue
- After the race, the runner continues to breathe deeply / takes in extra oxygen to break down / oxidise the lactic acid
Related
- Revision notes: Respiration →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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