Explain what happens to the bands a, i,h when aconraction occurs
in the muscles.

The functional unit of a muscle cell is known as a "sarcomere" which covers the distance between the two Z-lines.

The sarcomere consists of A-zone which contains both thick and thin filaments, H-zone which contains only thick filaments and I band which contains only thin filaments.

During muscle contraction, thick and thin myofibrils slide over each other due to which the distance between two z-line get reduced. The H and I zone gets shortened but A zone does not shorten but come close to other A- bands and disappears.

there are so many examples for that in different areas, like biology experiment carried out in our lab recently.

here's one link: