PURPOSE:To investigate the effect of physical therapy treatment on the timing of electromyographic (EMG) activity of the vasti in individuals with patellofemoral pain syndrome (PFPS). METHODS:Sixty-five (42 female, 23 male) participants aged 40 yr or less (29.2 +/- 7.8 yr) diagnosed with PFPS. Participants were randomly allocated into physical therapy treatment (McConnell-based) or placebo groups. Treatment programs were standardized and consisted of six-treatment sessions over 6 wk. Vastus medialis oblique (VMO) and vastus lateralis (VL) EMG activity was recorded with surface electrodes during a stair-stepping task and onsets of EMG activity were measured pre- and post-treatment. RESULTS:Before treatment, the EMG onset of VL occurred before that of VMO in both participant groups. After physical therapy intervention, there was a reduction in symptoms, and this improvement was associated with a significant change in the time of onset of VMO EMG compared with that of VL in both phases of the stair-stepping task. After physical therapy treatment, the onset of VMO preceded VL in the eccentric phase and occurred at the same time in the concentric phase of the stair-stepping task. There was no change in time of EMG onset in the placebo group. CONCLUSION:This study demonstrates that a "McConnell"-based physical therapy treatment regime for PFPS alters the motor control of VMO relative to VL in a functional task and this is associated with a positive clinical outcome.