# Skeletal muscle contraction (sliding filament model)

> Biology · CIE A-Level
> Source: https://www.owlsprep.com/study/cie-9700-u16-skeletal-muscle-contraction/

This module covers sarcomere structure, full cross-bridge cycling, roles of calcium and ATP, and sarcomere band changes, with targeted CIE 9700 exam response tips.

**Prerequisites:** [Structure of striated skeletal muscle fibres](https://www.owlsprep.com/study/cie-9700-u16-striated-muscle-structure/); [ATP structure and cellular energy roles](https://www.owlsprep.com/study/cie-9700-u8-atp-structure-and-function/)

## Learning objectives

- Identify key sarcomere structures and their roles in contraction
- Explain the full stepwise cross-bridge cycling process
- Relate calcium ion movement and ATP hydrolysis to contraction events
- Predict changes to sarcomere band patterns during full contraction

## Sarcomere Structure: Core Contractile Unit

A sarcomere runs between two adjacent Z-lines, and contains overlapping thick myosin filaments and thin actin filaments. All contraction events rely on the organised arrangement of these two filament types.

**Sarcomere** — The smallest functional contractile unit of striated muscle, whose shortening produces macroscopic muscle tension.

*Notation:* Z-line to Z-line

| Sarcomere Region | Composition |
| --- | --- |
| A-band | Full length of thick myosin filaments, including overlapping actin regions |
| I-band | Actin filaments not overlapping with myosin |
| H-zone | Myosin filaments not overlapping with actin |
| Z-line | Protein boundary that anchors actin filaments at each end of the sarcomere |

**Worked example:** A student is given an electron micrograph of a relaxed sarcomere 2.5 μm long, with A-band length 1.5 μm. Calculate the maximum possible length of the H-zone at full contraction.

1. First, note that A-band length never changes, so myosin filament total length is 1.5 μm.
2. Total sarcomere length at rest = 2.5 μm, so total actin filament length per side of Z-line = (2.5 - 1.5)/2 = 0.5 μm.
3. At full contraction, actin filaments fully overlap myosin, so H-zone length = 1.5 - (0.5 * 2) = 0 μm.

**Check your understanding**

1. Which structure anchors thin actin filaments at the edge of a sarcomere?

   - A-band
   - Z-line
   - H-zone
   - Troponin

   *Why:* Z-lines form the boundary of each sarcomere and attach actin filaments to keep them aligned.

## Stepwise Cross-Bridge Cycling

Cross-bridge cycling is the repeated sequence of attachment, movement, and detachment of myosin heads that pulls actin filaments over myosin to shorten the sarcomere. This process repeats as long as calcium ions and ATP are available.

> **mnemonic**
>
> Use the mnemonic CAT: Cock, Attach, Tug to remember the core sequence of myosin head actions during a single cycle.

**Worked example:** Order the following events of cross-bridge cycling correctly: 1. Power stroke pulls actin, 2. Myosin head hydrolyses ATP, 3. New ATP binds myosin head, 4. Myosin head binds actin

1. Step 1: ATP hydrolysis cocks the myosin head into an energised position: event 2 first.
2. Step 2: The energised myosin head forms a cross-bridge with the exposed actin binding site: event 4 next.
3. Step 3: Myosin releases ADP and Pi, pulling the actin filament in the power stroke: event 1 next.
4. Step 4: A new ATP molecule binds the myosin head, breaking the cross-bridge to reset the cycle: event 3 last.

**Exam command terms**

CIE uses specific command terms for this topic, with strict mark scheme requirements:

- **Describe** — List all 4 core cross-bridge steps in order, no extra detail required

- **Explain** — Include the role of ADP, Pi and ATP at each step to get full marks

- **Derive** — Link each cycle step directly to observed sarcomere shortening

## Role of Calcium Ions and ATP

Calcium ions released from the sarcoplasmic reticulum bind to troponin, causing troponin to change shape and move tropomyosin away from the actin binding sites that are normally blocked in resting muscle. Without calcium, no cross-bridges can form.

**Tropomyosin** — Fibrous protein that wraps around actin filaments, blocking myosin binding sites in the absence of calcium ions.

**Worked example:** A muscle fibre is placed in a solution with no calcium ions, but excess ATP. Predict what will happen when the fibre is electrically stimulated.

1. The action potential will travel down the T-tubules to the sarcoplasmic reticulum.
2. No extracellular calcium is available to enter the cytoplasm, so troponin remains bound to tropomyosin in the blocking position.
3. No myosin binding sites on actin are exposed, so no cross-bridges can form, and no contraction occurs even with abundant ATP.

> **tip**
>
> Exam mark schemes almost always award a mark for stating that calcium is actively pumped back into the sarcoplasmic reticulum using ATP to end contraction.

## Sarcomere Band Changes During Contraction

As actin filaments slide over myosin, regions of the sarcomere that contain only one filament type shorten, while regions that correspond to the full length of thick myosin filaments stay exactly the same length.

| Sarcomere Region | Change During Contraction |
| --- | --- |
| A-band | No change in length |
| I-band | Shortens significantly |
| H-zone | Shortens, and disappears at maximum contraction |
| Z-line to Z-line distance | Shortens overall |

> **warning**
>
> Never state the A-band shortens: this is the single most common lost mark on CIE 9700 muscle contraction questions.

## Common pitfalls

- **Wrong:** Stating the A-band shortens during contraction
  - Why it fails: The A-band corresponds to the full fixed length of thick myosin filaments, which never change size
  - Correct: Explicitly note only the I-band and H-zone shorten, while the A-band remains constant
- **Wrong:** Claiming ATP directly powers the power stroke
  - Why it fails: ATP hydrolysis resets the myosin head after the power stroke, while the power stroke itself releases stored ADP and Pi
  - Correct: Distinguish ATP binding (cross-bridge release) and hydrolysis (cock myosin) from the power stroke event
- **Wrong:** Saying tropomyosin binds calcium ions directly
  - Why it fails: Troponin is the dedicated calcium-binding protein, tropomyosin only moves after troponin changes shape
  - Correct: Specify Ca²+ binds troponin first, which then displaces tropomyosin from actin binding sites
- **Wrong:** Forgetting that muscle relaxation requires ATP
  - Why it fails: Many students only describe contraction, and omit the active transport of calcium back to the sarcoplasmic reticulum
  - Correct: Add that ATP-powered calcium pumps remove cytoplasmic calcium to end contraction and enable relaxation
- **Wrong:** Confusing the H-zone and I-band
  - Why it fails: Students mix up the two non-overlapping light regions of the sarcomere
  - Correct: Define the I-band as actin-only across two adjacent sarcomeres, and the H-zone as myosin-only at the centre of one sarcomere

## Cheatsheet

| Component | Behaviour during contraction | Key Exam Mark Point |
| --- | --- | --- |
| A band | No length change | Myosin filament length is fixed |
| I band | Shortens | Actin slides over myosin to reduce actin-only region |
| H zone | Shortens / disappears | Maximum overlap of actin and myosin at full contraction |
| Troponin | Binds Ca²+ to change shape | Calcium does not bind tropomyosin |
| Cross bridge | Forms, bends, releases, resets | Requires both Ca²+ and continuous supply of ATP |

## What's next

Mastering the sliding filament model is critical for scoring full marks on long answer physiology questions in CIE A-Level Paper 4, and it underpins your understanding of neuromuscular junctions, muscle fatigue, and movement disorders. You will next apply this knowledge to explore how action potentials from motor neurons trigger calcium release that initiates contraction, before comparing skeletal, cardiac and smooth muscle structure and function. This concept is also frequently tested in Paper 5 planning questions where you investigate the effect of stimulus intensity on muscle contraction force, so ensure you can link the molecular events you have learned to observable whole-muscle behaviour.

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