Final answer:
The H zone contains only thick filaments and shortens during muscle contraction, while the A band remains the same length.
Step-by-step explanation:
The H zone is a region within the sarcomere of a myofibril that is visible during muscle relaxation. It is located within the A band and, importantly, contains only thick filaments composed of myosin proteins. During muscle contraction, the H zone shortens as the actin filaments slide toward the M line, but the total length of the A band remains constant. The Z disc and M line help to anchor the myofilaments, maintaining the structural arrangement necessary for muscle contraction.