# A sprinter completes a 100-metre race and then continues to breathe heavily for several minutes afterwards. Explain why the sprinter's muscles switched to anaerobic respiration during the race and describe what happens in the muscle cells after the race to recover from this.

> Pearson Edexcel A-Level Biology A (9BI0) — 4.2 Muscle contraction · Explain · 5 marks

> During intense exercise, the cardiovascular system cannot always deliver oxygen to skeletal muscle cells fast enough to meet their energy demands.

## Mark scheme (5 marks)

1. During intense exercise, oxygen cannot be delivered to muscle cells fast enough / oxygen demand exceeds supply
2. So muscles respire anaerobically to continue producing ATP / energy for contraction
3. Anaerobic respiration produces lactic acid (lactate) as a waste product in the muscles
4. After the race, lactic acid must be broken down / oxidised, which requires extra oxygen — this is the oxygen debt
5. This is why the sprinter continues to breathe heavily after the race — to take in the extra oxygen needed to repay the oxygen debt

## Key terms

- [anaerobic respiration](https://www.gradenine.co.uk/glossary/anaerobic-respiration)

## 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)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/a-sprinter-completes-a-100-metre-race-3bbb65d5) · Published by Druglandscape Ltd.