The sliding filament model
Contraction increases the overlap between the actin and myosin filaments. Neither filament becomes shorter; instead, the thin filaments move further across the thick filaments.
Movement of actin towards the middle of the sarcomere reduces the distance between its Z-lines, so the entire sarcomere shortens.
| Feature | Change during contraction | Why |
|---|---|---|
| Z-lines | Move closer together | The sarcomere becomes shorter. |
| I band | Becomes shorter | A greater proportion of actin overlaps with myosin. |
| H band | Becomes shorter | Less of the myosin remains free from overlapping actin. |
| A band | Remains the same length | The length of each myosin filament is unchanged. |