Define the sliding filament theory of muscle contraction.

(N/A) The sliding filament theory explains the process of muscle contraction during which the thin filaments slide over the thick filaments,which shortens the myofibril.
Each muscle fibre has alternate light and dark bands,which contain special contractile proteins called actin and myosin,respectively.
Actin is a thin contractile protein present in the light band,known as the $I$-band,whereas myosin is a thick contractile protein present in the dark band,known as the $A$-band.
There is an elastic fibre called the $Z$-line that bisects each $I$-band. The thin filament is firmly anchored to the $Z$-line.
The central part of the thick filament that is not overlapped by the thin filament is known as the $H$-zone.
During muscle contraction,the myosin heads or cross-bridges come in close contact with the thin filaments.
As a result,the thin filaments are pulled towards the middle of the sarcomere.
The $Z$-line attached to the actin filaments is also pulled,leading to the shortening of the sarcomere.
Hence,the length of the $A$-band remains constant,while the $I$-band shortens and the $H$-zone disappears.