Relaxed sarcomeres contain myosin heads that have released their pull on the thin filaments. This allows them to return back to their relaxed state and causes the I bands and H zones to appear. The myosin heads pull on the thin filaments as a muscle contracts causing the disappearance of the I bands and H zones.
The relative movement of thick and thin filaments causes muscles to contract and relax. Groups of thick and thin filaments that alternately overlap and move apart are called sarcomeres. The areas between the thick and thin filaments during a relaxed state are called I bands, H zones and A bands. During contraction and relaxation, the length of the filaments remains the same. Myosin heads grasp the thin filaments and either push or pull them toward the thick filaments, causing the filaments to overlap or untangle and move apart.
Filaments are muscle fibers. Muscle fibers can be divided into three types. The skeletal muscle fibers move the bones and limbs. Smooth muscle fibers surround blood vessels and all organs except the heart. The heart contains cardiac fibers. Muscle fibers are integrated with other body systems to perform the function of muscle movement. Muscle movement requires the production of ATP.