Purpose] The purpose of our study was to measure the onset timing of the vastusmedialis oblique (VMO) relative to the vastuslateralis (VL) and the VMO/VL electromyographic (EMG) ratio in subjects with and without patellofemoral pain syndrome (PFPS) while they completed a functional task (stair stepping). [Subjects] Ten patients with PFPS and ten subjects without PFPS were included in this study. [Methods] The subjects performed stair stepping while surface electrodes were used to record the EMG

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... signal amplitude of the VMO and VL. [Results] During stair ascent and descent, significant differences in VMO/VL EMG ratios were observed between participants with and without PFPS. Significant differences were also observed for the VMO-VL timing among participants with and without PFPS while ascending and descending the stairs. [Conclusion] Our findings indicate that PFPS patients have a VMO/VL imbalance and delayed onset of VMO relative to VL.