Background: Flatfoot is a structural and functional abnormality of the foot that may cause lower limb mechanical damage during walking. The aim of this study was to investigate the acute effect of foot orthoses on the frequency domain of ground reaction forces in children with flatfeet during the stance phase of walking. Methods: Bilateral gait data were collected from fifteen male children suffering from flatfeet syndrome. Two Kistler force platforms were used to record the ground reaction forces of each limb during level walking. Arc support foot orthoses were used as an intervention. Results: No significant differences in the frequency content of the dominant limb ground reaction forces were found in the three directions in the two conditions (P > 0.05). However, the use of foot orthoses decreased nondominant limb medio-lateral ground reaction force frequency with a power of 99.5% (P = 0.015). Overall, for both limbs, the amplitude of the three-dimensional ground reaction force components during walking with foot orthoses were lower than those obtained without foot orthoses (P < 0.05). For both dominant and nondominant limbs, the essential number of harmonics in three directions during walking with and without foot orthoses were similar (P > 0.05). Conclusions: Foot orthoses decreased medio-lateral ground reaction force frequency in the non-dominant limb, but have little effect on other ground reaction frequency components. Decreased frequency phenomena due to foot orthoses in children with flatfeet suggest an attenuation of ground reaction forces during walking. Frequency domain analysis thus offered new insights on the gait improvements associated with using foot orthoses
