Final answer:
When a sarcomere contracts, the Z-lines move closer together due to the action of the sliding filament model of muscle contraction. This decreases the I band and H zone but the A band's length remains unchanged.
Step-by-step explanation:
When a sarcomere contracts, the Z-lines move closer together. This occurs because of the sliding filament model of muscle contraction. In this model, the thick filaments pull the thin filaments from both directions towards the center of the sarcomere, causing the Z-lines to draw closer to each other. As the sarcomere shortens, the I band, which contains only thin filaments, gets smaller, and the H zone, the part of the A band where there are only thick filaments, decreases in size. However, the overall length of the A band stays the same throughout the contraction. At full contraction, the thin filaments overlap more extensively.