The Tranversus Abdominis muscles are sometimes referred to as TVA or Corset Muscles. This is because they go all the away around like a corset and are deep inside. You cannot see them from the surface. When they work they act a bit like a corset but with the laces on the outsides. As there is a pull outwards on both sides what happens is that the front of your tummy will slide backwards becoming flatter.
