Final answer:
The A band is the region of the sarcomere that represents the overlap between thick and thin filaments. It remains at the same length during muscle contraction, while the degree of overlap between the filaments increases.
Step-by-step explanation:
The region of overlap between the two types of filaments in the sarcomere is the A band. The sarcomere is defined by the area from one Z line to the next, and it contains the entire A band. Within the A band, the thick (myosin) and thin (actin) filaments overlap, creating the dark appearance visible in the striation pattern of skeletal muscle. The A band does not change in length during muscle contraction; instead, the thin filaments slide past the thick filaments, increasing the overlap. The I band consists only of thin filaments and shortens during contraction, while the H zone, found within the A band, and containing only thick filaments, also shortens.