Each muscle fibre has many parallely arranged myofibrils. Each myofibril contains many serially arranged units called sarcomere which are the functional units. Each sarcomere has a central ' $\mathrm{A}$ ' band made of thick myosin filaments, and two half 'I' bands made of thin actin filaments on either side of it marked by ' $\mathrm{Z}$ ' lines. In a resting state, the edges of thin filaments on either side of the thick filaments partially overlap the free ends of the thick filaments leaving the central part of the thick filaments. This central part of thick filament, not overlapped by thin filaments is called the 'H' zone.