The sliding filament theory states that contraction of a muscle fibre occurs through the sliding of the thin filaments over the thick filaments. The thin filament is known as actin, and the thick filament is known as myosin.

• The contraction of the muscle is initiated when a neural signal is sent to the neuromuscular junction by a motor neuron by the central nervous system (CNS).

• In the muscle fibre it has an alternate light and dark bands which has the specialised contractile protein filaments called actin and myosin.

• An elastic fibre called Z-line is present in the muscle fibre which bisects each of the I-bands.

• The actin filament gets anchored to the Z-line. The central part of the myosin is not overlapped by actin filament, and it is known as the H-zone.

• During the muscle contraction, the head of the myosin filament come in close contact with the actin filament. This results in the actin filament to be pulled or slide towards the middle of the sarcomere.

• The Z-line which is attached to the actin filaments which leads to the shortening of the sarcomere.

• Thus, the length of the A-band would remain constant while the I-band shortens in length, and the H-zone gets reduced and ultimately disappears.

This mechanism occurs in all the sarcomeres results in the shortening of the myofibrils and thus causes muscle contraction.

The above diagram shows the contraction of a muscle fibre according to the sliding filament theory by the sliding of the thin filaments over the thick filaments.