A-band

The A-band is a region within muscle fibers, visible under a microscope, that contains the thick filaments made of a protein called myosin. It appears as a dark band due to the density of these filaments. The A-band's size remains constant during muscle contraction because the length of the thick filaments doesn’t change. It plays a key role in muscle contraction by providing the structure in which the thin actin filaments slide over the myosin, allowing muscles to shorten and generate force. In short, the A-band is essential for muscle movement, representing the core area where the contraction process occurs.