# A student is performing repeated bicep curls in a gym. During the first few repetitions, oxygen is available to the muscle cells and aerobic respiration occurs. As the student continues, the muscles begin to fatigue and lactic acid builds up. Explain how skeletal muscle cells obtain the energy needed for contraction during aerobic respiration, and explain what happens in the muscle cells and the body when lactic acid accumulates.

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

> Skeletal muscles require a continuous supply of ATP for contraction. During prolonged or intense exercise, the method of ATP production in muscle cells can change.

## Mark scheme (5 marks)

1. During aerobic respiration, glucose is broken down using oxygen to release energy / produce ATP
2. ATP provides the energy directly used for muscle contraction
3. When oxygen supply is insufficient, anaerobic respiration occurs in the muscle cells, producing lactic acid
4. Lactic acid causes muscle fatigue / pain, reducing the ability of muscles to contract effectively
5. After exercise, lactic acid is transported in the blood to the liver where it is broken down / oxidised, requiring extra oxygen — this is the oxygen debt

## Key terms

- [aerobic respiration](https://www.gradenine.co.uk/glossary/aerobic-respiration)
- [lactic acid](https://www.gradenine.co.uk/glossary/lactic-acid)

## 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-student-is-performing-repeated-bicep-15db53d6) · Published by Druglandscape Ltd.