Final answer:
During muscle contraction, the I band shortens while the A band stays the same length. Actin filaments anchor to Z lines. The zone of overlap between thick and thin filaments increases during muscle contraction.
Step-by-step explanation:
During muscle contraction, the I band of a sarcomere shortens, while the A band remains the same length. Actin filaments anchor to Z lines in a sarcomere. When a muscle contracts, the Z lines move closer together, causing the I band to get smaller. The A band, which contains both thick and thin filaments, stays the same width. As the thin filaments are pulled by the thick filaments, the zone of overlap between them increases.