The H-zone in the skeletal muscle fibre is due to:

• The absence of myofibrils in the central portion of A-band.
• The central gap between myosin filaments in the A-band.
• The central gap between actin filaments extending through myosin filament in the A-band.
• Extension of myosin filaments in the central portion of the A-band.

Within the A band, there is a relatively brighter central region, called the H-zone in which there is no actin / myosin overlap, when the muscle is in a relaxed state. When a muscle contracts, the actin is pulled along myosin toward the center of the sarcomere until the actin and myosin filaments are completely overlapped. The H zone becomes smaller and smaller due to the increasing overlap of actin and myosin filaments, and the muscle shortens. When the muscle is fully contracted, the H zone is no longer visible.