en Surgery Surgery Original Research: Clinical Science Original Research: Clinical Science Background: The purpose of this study was to investigate the ratio of Electromyography (EM G) activities of the vastus medialis obliqus (VMO) and vastus lateralis (VL) within seven angles of knee joint range of motion using isometric contraction in open kinetic chain (OK C) and closed kinetic chain (CKC). Methods: The dominant knees of 44 healthy female subjects (mean age: 24.84 with range: 20-30 years old) were analyzed. Surface electrode pairs were attached over the VMO and VL by using Basmajian methods. The EMG biofeedback data was collected while the subject performed isometric contraction on isometric and squat exercise within 7 degrees (0° to 90°) with 15° intervals of knee flexion angle. Results: Paired T-test andANOVA were performed between variables. Results demonstrated that in this study there are significant differences: I) between OKC and CKC exercise in 0°, 15°, and 30° ofknee flexion, II) between all angles in CKC except: 0-15°,0-30°, 0-45°, 15-30°, 60-75°, and 75-90°, and III) between 15-60°, 30-60°, 45-60°, and 60-90° in OKC (p<O.OOl). As a result in OKC and CKC respectively, 60° and 15° angles ofknee flexion were most sufficient positions (i.e. increased VMONL activities ratios), -15° and 90° ofknee flexion were the most insufficient positions and arc of movement between 45- 750 and 0-30° was considered as the most sufficient arc of movement. Conclusion: As the result of this research and biomechanical and neuro-physiological viewpoints, emphasized in the literature, our recommendations are: in early phase or in early sessions of treatment of patellofemoral joint dysfunction, treatment exercise done in CKC & sufficient position and after progression of the patient's condition, OKC exercises could also be considered. Patellofemoral Joint, Closed Kinetic Chain, Open Kinetic Chain, Electromyography 323 327 http://mjiri.iums.ac.ir/browse.php?a_code=A-10-298-15&slc_lang=en&sid=1 H FARAHINI 20031947532846003197 20031947532846003197 No GR SHAH HUSSEINI 20031947532846003198 20031947532846003198 No MJ SHATERZADEH 20031947532846003199 20031947532846003199 No I EBRAHIMI TAKAMJANI takamjan@tums.ac.ir 20031947532846003200 20031947532846003200 Yes School of Rehabilitation, Tehran University of Medical Sciences and Health Services, Tehran, Iran MS GHASEMI 20031947532846003201 20031947532846003201 No