Actin and myosin filaments slide over each other to increase the length of the sarcomere.
Length of A-band does not change.
I-band increases in length
The actin filaments slide away from A-band resulting in shortening of sarcomere.
Solution
According to Sliding filament theory, thin. Filament slide over Thick filament (mainly myosin). That is why the length of $\mathrm{A}$ band does not change as A band is made up of thick filament.