They align in adjacent myofibrils. I bands are areas near the edge of the sarcomere containing only thin filaments. A bands are regions where thick and thin filaments overlap and correspond to the length of the thick filaments. H zones are areas in the center of the A bands containing only thick filaments. The Mechanism of Muscular contraction .
Muscle contraction reduces the length of each sarcomere.
The sliding filament theory is explained below. The thin filaments ratchet across thick filaments to pull the Z lines together and shorten the sarcomere. The myofilaments themselves do not contract. Myosin molecules on thick filaments attach to the actin on the thin filaments to form a crossbridge. The cross bridges then bend inward pulling the thin filament toward the center of the sarcomere.