Background: Due to the fact that the lower limb is more in the closed movement chain, the position of the foot is an important and influential factor on the function of the lower limb(۱). Most functional disorders in the foot area are due to changes in the natural arches of the sole of the foot(۲). The foot, as a support for the body, is of special importance in the musculoskeletal system, which has three main functions: absorption of force, transmission of force and bearing of body weight(۳). The presence of arches in the sole of the foot provides the ability for the foot to perform its functions in a way that the most important of these arches is the internal longitudinal arch(۴). Disruption of the protective elements of the arches can cause abnormal positions of the flat foot and the sole of the foot(۲). Researchers have reported an association between impact forces transmitted to the spine, along with function and height of the longitudinal-internal arch of the foot, pelvic tilt, and subtalar pronation in walking(۵). Changes in the structure of the foot, such as the arch of the foot, can affect the lower limbs as well as the balance of the pelvis(۵). Biomechanical problems such as subtalar eversion, excessive overweight, plantar flexion in the talus, plantar flexion in the heel relative to the tibia, dorsiflexion and navicular abduction, anterior supination, and heel valgus (۶). Therefore, it is reasonable to assume that flat feet can cause other discomforts and disorders in the limbs, especially the lower limbs. In this case, it can be said that by treating this condition, we can improve and treat other disorders(۵).The aim of this study was to investigate the effect of foot pronation abnormalities along with overweight on electrical activity of vastus lateralis muscle during the loading response phase. Materials and methods: ۴۵ people in ۳ groups (۱۵ healthy people in control group ۱ and ۱۵ overweight people in the control group ۲ and ۱۵ overweight people with pronated feet in the experimental group) were purposefully selected. Bertec force plate system was used to record forces Ground reaction was used while walking. ANOVA was used for comparison between groups. Biometric EMG sustem was used to record EMG data. The significance level was set at P <۰.۰۵.).Results: The results showed that the frequency of vastus lateralis muscle activity during the loading response phase was higher in the group with pronated feet than in the healthy group (p = ۰.۰۲). Also, the median frequency of Vastus lateralis muscle activity during the loading response phase was lower in the overweight group with pronated foot than in the pronated group (p <۰.۰۱).Results :According to the research findings, it can be concluded that the activity of vastus lateralis muscle during the loading response phase in the overweight group with pronated feet is probably associated with changes compared to normal individuals. Therefore, it can be stated that the compensatory mechanism in the frequency of Vastus lateralis muscle occurs in overweight people with pronated feet foot to reduce the contact force.