OBJECTIVE Inappropriate neural control of the quadriceps femoris muscle group has been implicated in patellofemoral pain syndrome (PFPS). This study investigated the timing of initial electromyographic (EMG) activity of the vastus medialis oblique muscle (VMO) and the vastus lateralis muscle (VL) in asymptomatic subjects and subjects with PFPS during reflex and voluntary muscle activity. METHODS Fifteen symptomatic subjects (SYMP group) (9 with bilateral symptoms) and 12 asymptomatic subjects (ASYMP group) participated. Both knees were tested in the ASYMP group and only the symptomatic knees were tested in the SYMP group, resulting in a total of 24 data sets from each group. METHODS Electromyographic data were recorded from the VMO and VL under three conditions: reflex knee extension (RFLX) elicited by a patellar tendon tap, and active knee extension in non-weight-bearing (NWB) and weight-bearing (WB) situations. For each condition, EMG activity onset times for the VMO and VL were determined from ensemble averages of four trials. RESULTS There were no differences between the SYMP and ASYMP groups with respect to the relative timing of initial VMO and VL activity under any of the three conditions tested. Mean timing differences for both groups were less than 0.25 milliseconds under reflex conditions and less than 4 milliseconds for active knee extension under both WB and NWB conditions. CONCLUSIONS These findings contradict a previous report of differences in reflex timing related to PFPS. Differences in the relative timing of onset of EMG activity of the VMO and VL during voluntary knee extension were not significant between SYMP and ASYMP groups, and were not related to the relative timing differences observed during reflex testing.