Sarcomeres in different functional stages. In the resting state (middle), interdigitation of thin (actin) and thick (myosin) filaments is not complete; the H and I bands are relatively wide. In the contracted state (bottom), the interdigitation of the thin and thick filaments is increased according to the degree of contraction. In the stretched state (top), the thin and thick filaments do not interact; the H and I bands are very wide. The length of the A band always remains the same and corresponds to the length of the thick filaments; the lengths of the H and I bands change, again in proportion to the degree of sarcomere relaxation or contraction.
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