An article of footwear or shoe assembly, which is designed to allow for adjustment of stiffness in its sole unit, is disclosed. The article of footwear is preferably constructed to enhance stability, support and comfort of a wearer on varied terrain. Among other elements, the shoe assembly preferably includes an adjustable shank disposed within the shoe to allow for adjustment in the stiffness of the sole unit. In certain embodiments, the adjustable shank further includes a plurality of segments adapted to be manipulated in order to vary the stiffness.