# 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.

> Pearson Edexcel A-Level Biology A (9BI0) — Topic 5 Energy for Life · Explain · 5 marks

> 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.

## Mark scheme (5 marks)

1. Towards the end of the race the muscles cannot get enough oxygen to meet energy demands
2. So the muscles respire anaerobically (without oxygen)
3. Glucose is broken down to lactic acid (during anaerobic respiration)
4. The build-up of lactic acid causes the pain/fatigue in the muscles
5. 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

- [Revision notes for Pearson Edexcel A-Level Biology A (9BI0)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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