Short answer: during sarcomere contraction the I band shortens, while the A band remains essentially the same length. Explanation and details:

• Structure and bands
  • The sarcomere is the basic contractile unit of skeletal muscle. It contains alternating thick (myosin) and thin (actin) filaments. The light I bands contain thin filaments and are flanked by Z discs, while the dark A bands contain the full length of thick filaments and portions of thin filaments. The H zone is the central part of the A band where only thick filaments are present (no thin filaments). This organization is widely described in anatomy and physiology texts. [I band description and behavior: I-band contains actin; I-band shortens during contraction; A-band unchanged]

• What happens during contraction
  • As the muscle contracts, actin filaments slide inward toward the center (the M line) relative to the thick myosin filaments. This slide reduces the overlap distance in the I band and the H zone, causing the I band to shorten. The A band, which corresponds to the length of the thick filaments, remains essentially constant in length because the filaments themselves do not shorten. The net effect is shortening of the sarcomere with increased overlap between actin and myosin. [I band shortens; A band preserved in width]

• Related regulatory details (context)
  • The contraction process is driven by cross-bridge cycling: myosin heads bind exposed sites on actin in the presence of calcium, forming cross-bridges that pull actin inward. This mechanism underlies the shortening of the I band as actin filaments are pulled toward the M line. Regulatory proteins (troponin and tropomyosin) modulate access to actin binding sites in response to calcium, enabling contraction and relaxation cycles. While these regulatory details explain the mechanism, the key geometric change relevant here is the shortening of the I band with a stable A band. [Cross-bridge cycling and regulation]

If you’d like, I can add a concise diagram description or compare this behavior across different muscle types (e.g., skeletal vs cardiac) for clarity.