# 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.

> OCR GCSE Biology A: Gateway Science (J247) — B1.3 Respiration · Explain · 4 marks

> 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.

## Mark scheme (4 marks)

1. Towards the end of the race, the runner cannot supply oxygen to muscles fast enough / there is an oxygen debt
2. Anaerobic respiration occurs in the muscles, producing lactic acid
3. The build-up of lactic acid causes the muscle ache / pain / fatigue
4. After the race, the runner continues to breathe deeply / takes in extra oxygen to break down / oxidise the lactic acid

## Related

- [Revision notes for OCR GCSE Biology A: Gateway Science (J247)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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