When a skeletal muscle contracts which bands change in size and why?

During skeletal muscle contraction, the A (anisotropic) bands and the I (isotropic) bands change in size.

1. A Bands: The A bands shorten during contraction. This shortening is due to the sliding of the thin actin filaments into the spaces between the thick myosin filaments. As the actin filaments slide, they overlap more with the myosin filaments, increasing the number of potential cross-bridges and, thus, the force of contraction.

2. I Bands: The I bands also shorten during contraction. This shortening is caused by the pulling of the Z discs toward each other as the actin filaments slide. The shortening of the I bands allows the sarcomeres (the repeating units of skeletal muscle) to shorten, which ultimately results in muscle contraction.

In contrast to the A bands and I bands, the H (Hensen's) zones, M (middle) lines, and Z (Z discs) do not change in size during muscle contraction. The H zones represent the regions of the sarcomere where there is no overlap between actin and myosin filaments, while the M lines mark the center of the A bands. The Z discs serve as anchors for the thin actin filaments and maintain the structural integrity of the sarcomere.