The sliding filament theory proposes a mechanism of muscle contraction by which the arrangement of proteins in a muscle fibre brings about contraction.

It involves the actin and myosin filaments sliding past each other.

1. Ca²⁺ diffuses into the sarcoplasm, causing tropomyosin to pull away from and expose binding sites for myosin on actin molecules
2. Myosin heads form actin-myosin cross-bridges with actin filaments 
3. Myosin heads flex, pulling the actin filament along
4. ADP is released from myosin heads
5. ATP attaches to myosin heads, and myosin heads detach from actin
6. Myosin heads return to their original angle, using energy supplied by hydrolysis of ATP

1. Ca²⁺ is actively transported back into the sarcoplasmic reticulum
2. Tropomyosin moves back to block the actin filament
3. This prevents myosin heads from binding to actin filaments and the muscle relaxes

Energy for muscle contraction comes from the hydrolysis of ATP to ADP and inorganic phosphate.

This energy is used by the myosin head to return to its original position and for the reabsorption of calcium ions.

The myosin head can then attach itself to another actin-myosin binding site along the actin filament, repeating the cycle.

1. I band (light band) and H-zone become narrower as more of the actin filament is overlapping with the myosin filament
2. Z-lines move closer together
3. Sarcomere shortens

The A band (dark band) width remains the same.

Tropomyosin blocks the attachment site on the actin filament for myosin heads, preventing the formation of cross-bridges.

Myosin filaments have globular heads that are hinged, allowing them to move back and forwards, and the head has a binding site for actin and ATP.

The tails of several myosin molecules align together to form the myosin filament.

Actin is composed of two thin filaments that have binding sites for myosin heads, called actin-myosin binding sites.

These binding sites are usually blocked by another protein called tropomyosin.