ABSTRACT Background: Flatfoot deformity is a very common deformity among the musculoskeletal deformities, as twenty percent of the populations have a planus or low arched foot.Objective: to determine effect of flexible flat feet on electromyographic  activity of Erector spinae. Methods: forty subjects twenty with flexible flat feet and twenty normal people included in the study ; from students of the faculty of physical therapy ,Cairo University .they were assigned in two groups ; group A (flat foot  subjects ) group B (normal subjects ). Their mean ± SD age, weight, height and BMI were 23.75 ± 2.91 years , 74.85 ± 6 kg , 171.55 ± 5.07 cm, 25.51 ± 2.69 kg/m² and 23.15 ± 1.75 years , 73.2 ± 7.4 kg , 172.1 ± 4.01 cm, 24.73 ± 2.62 kg/ m². Navicualr drop test was used to assess subjects to determine who was flexible flat feet and who had not .Results : There was a significant increase in EMG activity of multifidus in group (A) compared with group( B ).Conclusion :Flexible flat feet  produce significant increase in  EMG activity of Erector spiane muscle compared with normal people who hadn’t flat feet.