Define the sliding filament theory of muscle contraction.

The sliding filament theory describes the process of muscle contraction in which the thick filaments (myosin) of muscle fibres slide past the thin filaments (actin), resulting in the shortening of the myofibril. Each muscle fibre consists of alternate light and dark bands. They contain a unique contractile protein called actin and myosin respectively. Myosin is a thick contractile protein found in the dark band and is known as the A-band or Anisotropic band. Actin is a thin contractile protein found in the light band and is known as the I-band or Isotropic band. Each I-band is bisected by an elastic fibre known as the Z line. The thin filament (actin) is firmly attached to the Z line. The H-zone is the central portion of the thick filament (myosin) that is not overlapped by the thin filament. Sarcomere, the functional unit of contraction, is the portion of the myofibril between two successive Z lines. During muscle contraction, the myosin heads come into close contact with the thin filaments. As a result, the thin filaments are pulled towards the centre of the sarcomere as well as the Z line attached to the actin filaments is also pulled, causing shortening of the sarcomere. The length of the A-band or anisotropic band remains constant as its original length whereas the I-band or isotropic band shortens and the H-zone disappears.