Final answer:
During muscle contraction, the H-zone and I-band of the sarcomere shrink, whereas the A-band's length remains unchanged. This process is part of the Sliding Filament Model of muscle contraction.
Step-by-step explanation:
During a muscle contraction, certain regions of the sarcomere change in length. Specifically, the H-zone and the I-band shrink. The H-zone is the part of the sarcomere that contains only thick filaments and shortens as the muscle contracts. Meanwhile, the I-band, which comprises solely thin filaments, also gets smaller. An important point to remember is that the A-band, representing the entire length of the thick filaments, remains unchanged in length during contraction, even though the sarcomeres themselves get shorter and the A-bands from neighboring sarcomeres come closer together. This shortening process is central to the Sliding Filament Model of muscle contraction, where thin filaments slide past the thick ones to shorten the sarcomere, thereby causing a muscle to contract.