Background: Flexible flat foot is a congenital common deformity in lower extremity associated with ligamentus laxity. This deformity may cause hammer toe, heel spur, inflammation of achille tendon, early muscle fatigue during sport, foot imbalance and pain in leg, knee and hip. The use of insole with medial arch support considered as one of the common treatments for patients. The purpose of this study was to determine the effect of a composite insole, Ethyl Vinyl Acetate (EVA) with Poron layer on ground reaction force on subjects with flexible flat foot in comparison with common EVA insole.

Materials and Methods: Fifty two subjects (14 Male and 38 female) between 18-40 years of age with flexible flat foot participated in this study. Twenty six subjects received Poron insole (EVA with Poron layer) and 26 subjects received common EVA insole. After assessment each subject asked to walk with normal speed across two force plate in two conditions, walking with insole and shoe and walking without insole. Then each subject asked to use insole for forty hours during two weeks. Vertical ground reaction force in the first session and after two weeks was assessed.

Results: Impact force and first peak of vertical ground reaction force (F₁) with Poron insole in comparison with shoe significantly increased in second session (P<0.05). The second peak (F₃) with common insole in comparison with shoe significantly increased in second session (P<0.05). Significant reduction was noted in first peak (F1) with common insole in second session in comparison with first session (P<0.05). There was no observable significant difference in two groups before and after intervention.

Conclusion: The results of this preliminary study show that both insoles change vertical ground reaction force in comparison with shoe alone. But using a composite Poron Layer on EVA insole has not demonstrated a significant difference with common arch support insoles. Therefore application of Poron Silicon layer on EVA insole by itself can not show a significant reduction of ground reaction force.