Final answer:
The sarcomere shortens during contraction as myosin pulls actin filaments inward, fueled by ATP. The H zone and I band decrease in length, and the thick and thin filaments overlap more, but the A band's length remains unchanged.
Step-by-step explanation:
During muscle contraction, the length of the sarcomere decreases. This is a result of the sliding filament model of muscle contraction where the myosin heads form cross-bridges with the actin filaments, pulling them towards the center of the sarcomere. The Z lines move closer together, the H zone and I band shorten, while the A band remains the same length. The overlap of the thin and thick filaments increases, which is essential for muscle contraction.
ATP is necessary for muscle contraction, providing the energy required for the myosin heads to bind to actin, pull the actin filaments inward, and then release and reset for another contraction cycle. The optimal length-tension relationship is achieved when there is maximal overlap of the myosin and actin filaments, allowing for the most cross-bridges to form and generate the greatest force.