When a muscle fiber contracts the you bands diminish in size h zones disappear and move closer together but do not length true or false?

False.

During muscle contraction, the thick filaments (myosin) slide past the thin filaments (actin) in a ratchet-like fashion, causing the Z discs to move closer together. This shortens the sarcomeres and results in the muscle fiber contracting. The H zones (the regions between the thick filaments) also shorten and eventually disappear as the thick filaments overlap more and more with the thin filaments. The A bands (the regions that contain both thick and thin filaments) remain the same length throughout contraction.