As explained by Amy Balsinger of Davidson University, sarcomeres are the basic contractile unit of muscles. Each muscle fiber contains hundreds of sarcomeres, which are repeating subunits of myofibril proteins. Muscles contract by sliding the thin actin filaments and thick myosin filaments along each other.
According to GetBodySmart, perpendicular protein plates called Z discs form the lateral boundaries on each side of a sarcomere. From the Z discs, thin actin filaments extend to the center of a sarcomere, where they partially overlap thick myosin filaments. Balsinger points out that the center of the sarcomere where only myosin resides during muscle rest is referred to as the H zone. During contraction, the overlapping thick filaments and thin filaments bind together and pull the Z disc towards the H zone in a ratchet-like manner. During a muscle contraction, the H zone disappears.