during muscle contraction in humans the?

During muscle contraction in humans, the A band remains of the same size. Increase in Ca⁺⁺ level into the sarcoplasm leads to the binding of calcium with a subunit of troponin on actin filaments and thereby remove the masking of active sites for myosin. Utilising the energy from ATP hydrolysis, the myosin head now binds to the exposed active sites on actin to form a cross bridge. This pulls the attached actin filaments towards the centre of ‘A’ band. The ‘Z' line attached to these actins are also pulled inwards thereby causing a shortening of the sarcomere, i.e., contraction. It is clear from the above steps, that during shortening  of the muscle, i.e., contraction, the ‘I’ bands get reduced, whereas the ‘A’ bands retain the length.