First let us see what Z line, H zone, I band and A band are.

A band -
It is a dark band present on myofibril. It is also known as anisotropic band.

I band
It is a light band present on myofibril. It is also known as isotropic band.

H band
It is a ligher area present at the centre of A band. It also known as Hensen's zone.

Z line
It is a dark line that passes through I band. It is also known as Zwischenscheibe line.

During muscle contracting, muscle fibres shorten, hence, -

1. Z line - pulled inwards (hence sarcomere shortens)
2. H zone - narrows
3. I band - length gets reduced
4. A band - length remains unchanged

Hloo


Upon muscle contraction, the A-bandsdo not change their length (1.85 micrometerinmammalian skeletalmuscle), whereas the I-bandsand theH-zoneshorten. This causes the Z lines to come closer together. The protein [tropomyosin] covers the myosin binding sites of the actin moleculesinthemusclecell.

Good luck!