Compare these electron micrographs. One is of a relaxed muscle and the other of a contracted muscle.

When a muscle contracts, the sarcomere shortens which means the whole muscle shortens. However, there’s no change in length of the thick filament (the dark band). This means that instead of contracting, filaments slide past each other instead. Hence the sliding filament theory.

The cross-bridge cycle

In order for the filaments to slide past each other energy is required. The ATPase needed to split the ATP is contained within the myosin cross-bridge head. The cross-bridge attaches to the actin and ‘walks’ along it, a process known as the cross-bridge cycle.

The cycle consists of four steps:

• The cross-bridge attaches to the thin filament by swinging out from the thick filament.
• The cross-bridge then changes shape and turns at 45