Final answer:
During muscle contraction, the A band of the sarcomere remains unchanged in length while the sarcomere itself shortens as the thin filaments slide over the thick filaments.
Step-by-step explanation:
During muscle contraction, the length of the region of the sarcomere that stays unchanged is the A band. The A band is composed of thick myosin filaments and it maintains a consistent length as muscles contract. On the other hand, the I band and the H zone, which are composed of thin actin filaments and the central region of the A band respectively, shorten during contraction. The sliding filament model of muscle contraction explains this process, wherein the thin filaments slide over the thick filaments, causing the sarcomere to shorten and generate tension without changing the length of the A band.