A sprinter finishes a 100 m race and immediately feels their leg muscles become stiff and sore. Explain how ATP is used during muscle contraction and describe what happens when ATP supply cannot meet the demand of contracting muscle fibres.

Edexcel A-Level Biology A: Salters-Nuffield (9BN0) — 4.2 Muscle contraction · Explain · 5 marks · View as Markdown

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

Skeletal muscles are made of muscle fibres containing overlapping actin and myosin filaments. Contraction requires a continuous supply of ATP. During intense, short-duration exercise such as sprinting, the muscles may not receive sufficient oxygen to produce all the ATP needed.

Model answer (5 marks)

ATP is hydrolysed by ATPase to ADP + Pi, releasing energy that powers the myosin head to bind to actin and pull the actin filament, causing a power‑stroke.

During intense sprinting the oxygen supply is insufficient for aerobic ATP production, so the muscle relies on anaerobic glycolysis.

Anaerobic glycolysis produces lactate (lactic acid) from pyruvate.

The lactate accumulates, lowering the pH of the muscle.

The acidification and depletion of ATP lead to muscle fatigue, pain and cramps, reducing the ability of the fibres to contract.

Examiner tips

  • Use the term ‘ATPase’ and ‘hydrolysis’ to show understanding of the energy source. Explain the shift to anaerobic glycolysis when oxygen is limited. Mention lactate formation and its effect on pH. Show the link between low ATP, acid build‑up and reduced contraction ability.
  • common_mistakes
  • :
  • Confusing ATP with ADP or omitting the ATPase step. Failing to mention anaerobic glycolysis or lactate production. Using vague terms like ‘energy’ without specifying the biochemical process. Neglecting the link between low ATP, acidification and muscle fatigue.

Mark scheme (5 marks)

  1. ATP provides energy for the myosin head to move / bind to actin and pull the actin filament
  2. ATP is broken down (by ATPase) to ADP and inorganic phosphate, releasing energy
  3. When oxygen supply is insufficient, anaerobic respiration occurs in the muscle cells
  4. Anaerobic respiration produces lactic acid (lactate) in muscle cells
  5. Accumulation of lactic acid causes muscle fatigue / pain / cramp, reducing the ability of muscles to contract

Key terms in this question

ATP · actin

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