The following events take place during muscular contraction.
(1) Myosin hydrolysis ATP into ADP and Pi. The energy released is used to make a cross-bridge with actin filaments. This pulls the actin filaments towards A-band.
(2) 'Z' line attached to these actins are also pulled inwards thereby causing the shortening of the I-band and thus the sarcomere.
(3) The size of the A-band remains constant.
(4) The movement of actins towards the A-band, causes H-zone to disappear.