# A student runs a long-distance race and towards the end feels a painful sensation in their leg muscles, forcing them to slow down. Explain why this pain occurs during vigorous exercise.

> Cambridge International IGCSE Biology (0610) — 12.3 Anaerobic respiration · Explain · 4 marks

## Mark scheme (4 marks)

1. Muscles do not receive enough oxygen during vigorous exercise
2. Muscles respire anaerobically (without oxygen)
3. Lactic acid is produced as a product of anaerobic respiration in muscles
4. Build-up of lactic acid causes muscle fatigue / inhibits anaerobic respiration / causes cramp

## Related

- [Revision notes for Cambridge International IGCSE Biology (0610)](https://www.gradenine.co.uk/learn)
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