: Titin (a large protein) forms a third filament system, allowing muscle to spring back into shape after being Stretched. It reaches from the Z line to the M line, with a single filament measuring over 1 mm in length. Part of the molecule overlaps the A band (where actin and myosin overlap) and part overlaps the I band (mainly actin). Thick filament, confined to the A band, contains chiefly the protein myosin. The thin filament, lies in the I band and extends into the A band but not into its H zone.