Objective: Morphological changes of the abductor hallucis muscle (AbH) in flexible flatfoot (FF) individuals influence regulations of the medial longitudinal arch (MLA). Prolonged and repeated stretching of AbH in flexible flatfoot may cause changes in muscle reflex properties and further influence postural performance. However, AbH muscle reflex under different postural conditions have never been examined. The purpose of this study was to investigate differences in AbH H-reflex and postural performance between individuals with normal foot (NF) alignment and FF under prone, double-leg stance (DLS), and single-leg stance (SLS) conditions. Design: Cross-sectional study. Setting: University laboratory. Participants: Individuals with FF (n = 12) and NF (n = 12). Main outcome measures: AbH H-reflex, AbH EMG and center of pressure (CoP) displacement. Results: Under all postural conditions, AbH H-reflex was significantly lower in the FF group (P <.05). Under the SLS condition, AbH EMG was significantly higher in the FF group (P <.05), and CoP displacement for the medial-lateral and anterior-posterior directions were significantly higher in the FF group (P <.05). Conclusions: With increased postural demand, FF individuals maintained their postural stability by recruiting greater AbH activities than through automatic stretch reflex, but FF individuals still showed inferior posture stability.