# A long-distance runner completes a marathon. Towards the end of the race, the runner's muscles begin to ache and feel heavy. After the race, the runner breathes rapidly and deeply for several minutes before their breathing returns to normal. Explain why the runner's muscles ache during the race and why the runner continues to breathe rapidly after the race has finished.

> Edexcel A-Level Biology A: Salters-Nuffield (9BN0) — 4.1 Cellular respiration · Explain · 5 marks

> A long-distance runner completes a marathon. Towards the end of the race, the runner's muscles begin to ache and feel heavy. After the race, the runner breathes rapidly and deeply for several minutes before their breathing returns to normal.

## Mark scheme (5 marks)

1. Towards the end of the race, oxygen supply to muscles is insufficient / muscles cannot get enough oxygen
2. Muscles respire anaerobically (as well as aerobically)
3. Anaerobic respiration produces lactic acid, which causes the muscle ache / fatigue
4. After the race, the runner has an oxygen debt that must be repaid
5. The extra oxygen is used to break down / oxidise the lactic acid (so breathing remains rapid until the debt is repaid)

## Related

- [Revision notes for Edexcel A-Level Biology A: Salters-Nuffield (9BN0)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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