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Purpose: Less attention has been paid to the electromyographic activity of the lower extremity muscles, which is considered as an essential part of the kinetic studies on the gait of mentally retarded individuals. Hence, the study aims at determining the surface electromyography characteristics of the lower extremity muscles of mentally retarded adolescents during walking. Methods: It is a causal-comparative study. Fifteen mentally retarded and 15 normal adolescents with an age range of 10 to 14 years participated in this study. To record the activities of vastus medialis, vastus lateralis, biceps femoris, semi-tendinosus, tibialis anterior, long peroneal, medial gastrocnemius, and soleus muscles, sEMG was employed during the stance phase of gait. For the data analysis, an independent sample t-test was conducted using SPSS version 18. Results: The results revealed that the mentally retarded adolescents had higher level of biceps femoris muscle activity in the heel contact sub-phase (P=0.016) compared to the normal group. Also, the vastus medialis (P=0.015) and the long peroneal (P=0.026) muscles showed higher EMG activity. Furthermore, their vastus lateralis (P=0.039) and Soleus (P=0.002), and vastus medialis (P=0.045) muscles demonstrated higher and lower activities, respectively. The co-contraction rate of medial gastrocnemius and anterior tibialis muscles during the heel contact was higher (P=0.040) in individuals with mental retardation compared to the healthy group. Conclusion: It can be concluded that the mentally retarded individuals use different muscle activation patterns in comparison to healthy people. As a result, special attention should be given to the functioning of their lower extremity muscles during the corrective power exercises.