During muscle contraction, which band remains the same length?

The A band remains the same length during muscle contraction. The A band is composed of thick filaments (myosin) and overlaps with thin filaments (actin), and it corresponds to the length of these thick filaments. During contraction, the actin filaments slide over the myosin filaments, which leads to a shortening of the sarcomere, but the length of the thick filaments (and thus the A band) does not change.

In contrast, the I band, which contains only thin filaments, decreases in length as the muscle contracts; the H band, which is the region where thin filaments do not overlap with thick filaments, also diminishes in size. The Z line, which is the boundary between adjacent sarcomeres, moves closer together during contraction, further emphasizing that the regions outside the A band change in length. Therefore, the property of the A band remaining constant during contraction is essential for understanding the mechanics of muscle function.