<P> The myosin head now binds to the actin myofilament . Energy in the head of the myosin myofilament moves the head, which slides the actin past . ADP is thereby released . </P> <P> ATP presents itself (as the presence of the calcium ions activates the myosin's ATPase), and the myosin heads disconnect from the actin to grab the ATP . The ATP is then broken down into ADP and phosphate . Energy is released and stored in the myosin head to utilize for later movement . The myosin heads now return to their upright relaxed position . If calcium is present, the process is repeated . </P> <P> 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 . </P>

List the filaments in order of increasing size