Bryan Gothie wrote an answer about New Balance 978 Hiking Boot - Men's on December 15, 2010
The New Balance 978 provides moderate arch support. There is no traditional medial post but there are several other features which combine to provide both stability and support. There is a TPU plate called Rockstop whihc runs from the forefoot to the midooft that provides the majority of stability. In addition, the Rollbar in the heel includes bilateral pieces of TPU that rest on a piece of graphite. The combination of these three pieces provide plenty of support for your heel.
So will there is nothing in the shoe that is directly acting on your arch, the overall platform of the shoe has been built to provide stability and support.
-Bryan Gothie, NB Product Manager