Explain how the length–tension relationship of a sarcomere determines the force a skeletal muscle can generate at different degrees of stretch.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Skeletal muscle generates maximum force only within a specific range of sarcomere lengths. When a muscle is either overstretched or compressed beyond this optimal range, force production declines.
Model answer (4 marks)
At the optimal sarcomere length (≈2.0–2.4 µm) actin and myosin filaments overlap maximally, allowing the greatest number of cross‑bridges to form and thus maximum force.
If the sarcomere is overstretched, the overlap between actin and myosin decreases (or disappears), so fewer cross‑bridges can form and force falls.
If the sarcomere is excessively shortened, actin filaments from opposite Z‑discs overlap or myosin heads contact the Z‑disc, sterically hindering cross‑bridge cycling and reducing force.
Hence the length–tension relationship is an inverted‑U (bell‑shaped) curve: low force at very short and very long lengths, with a plateau of maximum force at intermediate, optimal lengths.
If the sarcomere is overstretched, the overlap between actin and myosin decreases (or disappears), so fewer cross‑bridges can form and force falls.
If the sarcomere is excessively shortened, actin filaments from opposite Z‑discs overlap or myosin heads contact the Z‑disc, sterically hindering cross‑bridge cycling and reducing force.
Hence the length–tension relationship is an inverted‑U (bell‑shaped) curve: low force at very short and very long lengths, with a plateau of maximum force at intermediate, optimal lengths.
Examiner tips
- Use the terms ‘optimal sarcomere length’, ‘cross‑bridge formation’, ‘overlap’, ‘inverted‑U curve’ – these are the key phrases examiners look for.
- Show the three regimes (short, optimal, long) in a clear, concise paragraph – 4 marks are awarded for covering all three.
Mark scheme (4 marks)
- At optimal sarcomere length, maximum overlap between actin and myosin filaments allows the greatest number of cross-bridges to form simultaneously, producing maximum force.
- When a sarcomere is overstretched, actin and myosin filaments overlap less (or not at all), so fewer cross-bridges can form, reducing force production.
- When a sarcomere is excessively shortened/compressed, actin filaments from opposite Z-discs overlap with each other (or myosin contacts Z-discs), sterically hindering cross-bridge cycling and reducing force.
- The relationship is therefore described as an inverted U-curve (or bell-shaped curve): force is low at very short and very long lengths, with a plateau of maximum force at intermediate (resting/optimal) sarcomere lengths of approximately 2.0–2.4 µm in human skeletal muscle.
Key terms in this question
sarcomere · length–tension relationship
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
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