As per sliding filament theory, muscle contraction takes place because of sliding of actin and myosin filaments towards each other. When actin and myosin filaments slide away from each other, relaxation of muscles happens. In striated muscles, the striations appear because of alternate bands of actin and myosin. The band of actin is light in colour and is called the I band. The myosin band is darker in colour and is called the A band. The actin filaments are held in the middle by an elastic band called Z line. The myosin filaments are held in the middle by an elastic band called M line. During contraction, position of Z line changes in relation to M line and muscle fibre becomes shorter. During relaxation, the actin filaments move to their original position and the muscle fibre appears longer.