I band is the lighter and a less dense area of the sarcomere. It contains no thick filaments. A Z disc passes through the center of each I band. During muscle contraction myosin heads become attach to and slide along the thin filaments at both ends of a sarcomere. It pulls the thin filaments toward the M line and makes the thin filaments slide inward and meet at the center of a sarcomere. This inward movement of thin filament allows their ends to overlap.
As the thin filaments slide inward, I band and H zone narrower. I band and H zone become disappear altogether when the muscle is maximally contracted. The width of the A band and the lengths of the thick and thin filaments remain unchanged.
