A mountaineer climbing at high altitude notices that their muscles become fatigued and painful much more quickly than when climbing at sea level. Explain why the mountaineer's muscles fatigue more quickly at high altitude.

WJEC GCSE Biology (Wales) — 1.2.1 Aerobic and anaerobic respiration · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

At high altitude, the concentration of oxygen in the air is much lower than at sea level.

Model answer (4 marks)

At high altitude there is less oxygen available to the muscles.
Because of the low oxygen, the muscles cannot carry out aerobic respiration efficiently and therefore switch to anaerobic respiration to meet the energy demand.
Anaerobic respiration produces lactic acid in the muscles.
The build‑up of lactic acid causes muscle fatigue and pain.

Examiner tips

  • Use the exact sequence: low O₂ → anaerobic switch → lactic acid → fatigue. Show cause–effect links. Include the word "anaerobic" and "lactic acid" to gain full marks.

Common mistakes

  • Saying only that muscles use less ATP without mentioning anaerobic respiration. Forgetting to mention lactic acid as the cause of fatigue. Using vague terms like "low oxygen" without linking to anaerobic metabolism.

Mark scheme (4 marks)

  1. At high altitude there is less oxygen available to the muscles
  2. The muscles switch to anaerobic respiration (because aerobic respiration cannot meet the energy demand)
  3. Anaerobic respiration produces lactic acid in the muscles
  4. The build-up of lactic acid causes muscle fatigue / pain

Key terms in this question

fatigue

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