Figure 18-13 The basic unit for contraction is the sarcomere. A contracted and a relaxed sarcomere is depicted. Z lines are located at the ends of the sarcomere. The A band is the site of overlap between myosin and actin filaments. The I band is located on either side of the A band and contains only actin filament. The H zone is located in the center of the A band, and only myosin is present.