According to the sliding filament theory:
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
According to the sliding filament theory, muscle contraction occurs when thin filaments (actin) slide over thick filaments (myosin). This pulls the actin filaments towards the centre of the 'A' band, pulling the 'Z' lines inwards and shortening the sarcomere. During this process, the 'I' bands get reduced in length, whereas the 'A' bands retain their length.
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