Question

What happens to the I band and H zone during contraction? Do the lengths of the thick and thin filaments change?

Answer 1

The formation of the cross-bridges pulls the actin filament towards the centre of the A-band. The Z-line attached to this actin is also pulled inwards thereby causing a shortening of the sarcomere i.e. contraction occurs. The thin myofilaments move past the thick myofilaments due to which the H- zone narrows. This, in turn, reduces the length of the I band but retains the length of A band.

The thick and the thin filaments refer to the actin and myosin filaments respectively. No, their lengths do not change but instead, they slide over each other.