A bands and I bands

- At the centre of each A band is a lighter-coloured region called the H-zone - At the centre of each I band is a line called Z-line - The distance between adjacent Z-lines is called sacromere - When muscle contracts, these sacromeres shorten + the pattern of light + dark bands changes During contration - Z lines = closer together - H zone = shortens - I band = shortens - A band = stays the same