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A muscle shortens because filaments slide past one another, not because actin and myosin filaments become shorter. Connect calcium regulation, ATP cycling and whole-muscle action.
Video by Cognito. Foundation recap: the written lesson develops the A-level detail.
Viewing focus: Explain sarcomere changes during contraction
After watching, attempt these questions before opening the answers. You can also use the written lesson below.
After watching 1: During sliding-filament contraction, the A band is approximately:
Unchanged in length. It corresponds to thick-filament length.
After watching 2: ATP binding to myosin is important for:
Detachment from actin. Hydrolysis then helps reset the head.
What you'll be able to do
Skeletal muscle contains bundles of muscle fibres, each containing many myofibrils. Repeating sarcomeres contain thin actin and thick myosin filaments arranged between Z lines. During contraction, overlap increases and Z lines move closer together; filament lengths remain essentially unchanged.
The A band corresponds to the thick-filament length and stays approximately constant, while the I band and H zone narrow in a contracting sarcomere. Muscles pull rather than actively push, so antagonistic muscle pairs act around joints against a supporting skeleton. Explain which muscle contracts and which relaxes in a specified movement.
Muscle electrical activity causes calcium release from internal stores. Calcium-dependent changes move tropomyosin away from myosin-binding sites on actin, allowing cross-bridges to form. Calcium binding to troponin changes the arrangement of tropomyosin, exposing myosin-binding sites on actin.
An energised myosin head binds actin and undergoes a power stroke. ATP binding permits detachment from actin; ATP hydrolysis re-energises the head for another cycle. Calcium removal helps restore the blocking position and relaxation. ATP is also needed for ion handling, so lack of ATP cannot be described simply as a normal relaxed state.
Phosphocreatine can rapidly donate phosphate to ADP to regenerate ATP during short periods of high demand. Its store is finite and must be replenished, so it is not a permanent replacement for respiratory ATP production.
Slow fibres generally have many mitochondria, good capillary supply and substantial myoglobin, supporting prolonged aerobic activity and fatigue resistance. Fast fibres can generate rapid, powerful contractions and often rely more on rapid glycolytic ATP supply, with greater fatigue in the simplified comparison. Fibre properties vary, so use information supplied about a particular subtype rather than treating every fast fibre as identical.
Try each question before opening the worked response. Use precise biological terms, connect each cause to its effect, and check units and assumptions when interpreting data.
Common mistakes to avoid
Key takeaways
Check the assessed scope and practical or mathematical requirements.
An external explanation or interactive activity to use alongside the lesson. It may include extension material.
Close the notes and answer from memory. Revisit any explanation you cannot yet reconstruct.
Test yourself
Answer a few questions, then review anything you missed.
Start with the quiz above to check the key ideas. You can also choose another topic from the practice library.