'A' band is the region of striated muscle fibre that contains both thick (myosin) and thin (actin) filaments. It is visible as a dark band with a lighter central zone (H zone) in the middle of a sarcomere. During muscle contraction the thin actin myofilaments slide past the thick myosin filaments to penetrate deeper into 'A' band thereby shortening the I bands and sarcomere and also the muscle fibre. During relaxation the actin myofilaments slide back from A bands thus elongating the sarcomere. Thus, A band remains unchanged during contraction and relaxation of the skeletal muscle.