Fig. 8.

(a) A simplified schematic of two convergent forces (F1 and F2) applied on the same solid will produce a movement in the direction of their resultant (R); (b) schematic representation of the diadochal movements where the action of the triceps surae (F1) and posterior tibialis (F2) will produce a varus position of the talocalcaneonavicular (TCN) joint with regard to the forefoot. Subsequently an intrinsic deformity of the calcanealpedal unit is noted with an adduction-supination of the forefoot.