Objective To investigate the possible effect of fully knee extension in sitting position with or without hip adduction on quadriceps imbalance in patients with patellofemoral pain syndrome (PFPS) by using surface electromyography (sEMG). Methods sEMG signals of vastus lateralis (VL) and vastus medialis oblique (VMO) muscles from 30 patients with PFPS as PFPS group and 30 healthy subjects as control group were collected, during their fully knee extension in sitting position with or without hip adduction. All the EMG data were then analyzed by extracting time domain indexes, namely, the root mean square (RMS) and integrated EMG (IEMG), to compare the balanced relationship between VL and VMO muscles. Results In PFPS group, there were no significant differences in VL time domain indexes during knee extension in sitting position with or without hip adduction, while statistical differences were found in VMO time domain indexes, and values of RMS and IEMG of VMO were higher under hip adduction, which indicated that the VMO muscle recruitment was strengthened. Conclusions Fully knee extension in sitting position with hip adduction can promote EMG activities of VMO muscles in patients with PFPS, which will help to balance the VL and VMO muscles.