Structurally the foot is equivalent to a twisted plate. The hindpart is located in the sagittal plane and the forepart in the transverse plane. The transition induced by the twist creates the transverse and the longitudinal arches. Under vertical loading of the foot plate by the tibiotalar column, compressive forces are created on the dorsum and tensile forces on the plantar aspect of the foot plate. The plantar aponeurosis acting as a tie-rod when under tension relieves the tensile forces from the plantar aspect of the foot plate. The increased tension in the plantar aponeurosis in the weightbearing position of the foot occurs with anterior flexion of the leg or with hyperextension of the toes. In the plantigrade position when vertical loading and external rotation are simultaneously applied by the tibiotalar column on the foot, the hindfoot and the midfoot are supinated, and the forefoot is pronated. The medial longitudinal arch is higher, the foot is shorter, and the plantar aponeurosis is relaxed. The foot is then more flexible. With vertical loading and simultaneous internal rotation, the hindfoot and the midfoot are pronated, and the forefoot is supinated. The medial longitudinal arch is lower, the foot is longer, and the plantar aponeurosis is tense. The foot is then more rigid and a better lever arm. Demonstrations are presented both in living and in anatomic dissections.
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