Kevin A. Kirby, DPM, MS*
A new theory of foot function based on the spatial location of the subta- lar joint axis in relation to the weight bearing structures of the plantar foot is proposed. The theory relies on the concept of subtalar joint rotational equilibrium to explain how externally generated forces, such as ground reaction force, and internally generated forces, such as ligamentous and tendon tensile forces and joint compression forces, affect the mechani- cal behavior of the foot and lower extremity. The biomechanical effect of variations among individuals in the spatial location of the subtalar joint axis are explored, along with their clinical consequences, to offer an ad- ditional theory of foot function, which may improve on existing podiatric bio- mechanics theory. (J Am Podiatr Med Assoc 91(9): 465-487, 2001)