Browsing through this site I found an article, “Ball props on the reformer: A help or hinder’ by Madeleine Black. This nicely revisits the work she shared at the APMA Sydney retreat in February 2010.
” the use of the ball inhibits functional synergistic muscle work of the feet up to the spine. We are looking at optimal leg alignment while executing Pilates in a dynamic and functional way, not in a held position.”
is the most recent writer I have seen explore this part of the body from our perspective. 
