When a muscle contracts, the actin is pulled along myosin toward the center of the sarcomere until the actin and myosin filaments are completely overlapped. The H zone becomes smaller and smaller due to the increasing overlap of actin and myosin filaments, and the muscle shortens. Thus when the muscle is fully contracted, the H zone is no longer visible. Note that the actin and myosin filaments themselves do not change length, but instead slide past each other. This is known as the sliding filament theory of muscle contraction.