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Objective: Exercise therapy to strengthen quadriceps muscle is recommended in rehabilitation program for people with patellofemoral pain syndrome (PFPS). This study aimed to investigate the electromyography (EMG) activity of vastus medialis obliquus (VMO), vastus lateralis (VL) and VMO/VL ratio during PNF in individuals with and without PFPS., Methods: 26 persons with PFPS and 26 healthy subjects participated to study. All subjects performed PNF patterns (Flexion-Adduction-External Rotation (D 1 FL), Extension-Adduction-External Rotation (D 2 EX), D 1 FL+ load, D 2 EX+ load) and straight leg raise (SLR). The normalized EMG activity of VMO, VL and VMO/VL ratio were measured and analyzed using repeated measure ANOVA., Results: There were significant main effects of group and exercises for the both VMO and VL (p<0.05). It was found that except SLR and D 2 EX, in the other motions PFPS group had lower VMO activity compared to healthy group (p<0.05). For VL except SLR, in the other motions PFPS group had lower VL activity too (p<0.05). The PNF patterns activated VMO more than SLR, however it was not significant (p>0.05). Also; there weren't any significant difference between the two groups in VMO/VL activation ratios. Also, performing the PNF patterns with load increased VMO and VL muscles activity significantly (p<0.05). It also found that in PFPS group the VMO/VL ratio values in PNF patterns were significantly more than SLR and the highest VMO/VL ratio value (0.96) was found during D 2 EX., Conclusion: The PNF patterns due to provide optimal VMO/VL ratio value than SLR and proper balance between these two muscles can be recommended in rehabilitation of individuals with PFPS., Competing Interests: There is no conflict of interest to disclose., (2021, JAPANESE PHYSICAL THERAPY ASSOCIATION.)