Medial vasti insufficiency is commonly accepted as being responsible for lateral patellar tracking, however, electromyographic (EMG) studies supporting this hypothesis are inconclusive. With the advent of kinematic magnetic resonance imaging (KMRI), it was the purpose of this study to establish the role of the vasti in contributing to patellar kinematics. Additionally, gait analysis was performed to assess vasti activation patterns in this population and to identify compensatory gait mechanisms characteristic of this disorder. Twenty-six female patients with patellofemoral pain (PFP), and 19 controls participated in this study. Subjects underwent KMRI of the patellofemoral joint using a device that allowed resisted knee extension from 45 to 0 degrees. Following KMRI, EMG analysis (indwelling electrodes) of the vastus lateralis (VL), vastus medialis longus (VML), vastus medialis oblique (VMO) and the vastus intermedius was conducted using the sam e apparatus. Gait analysis then was performed with EMG, joint motion (Vicon), stride characteristics (Footswitch Stride Analyzer), and force plate data (Kistler) being collected simultaneously during level walking, stairs and ramps. KMRI data were assessed for medial/lateral patellar displacement, tilt, and sulcus angle at six positions of knee flexion. Normalized VL:​VMO and VL:​VML EMG ratios were calculated at the sam e angles. EMG timing during gait was determined by footswitches. Stepwise regression was employed to determine if the sulcus angle or EMG ratios were predictive of patella tracking patterns, while two-way Anova’s (repeated measures) were utilized to compare gait data between groups and conditions. The sulcus angle was predictive of lateral patellar tilt and displacement at 0 degrees, supporting the premise that bony structure is the primary determinate of patellar stability. The VL:​VML EMG ratio was inversely related to patella malalignment suggesting recruitment of the VML was in response to, and not a cause of abnormal tracking. During gait, there were no EMG differences indicative of compromised patellar stability, however, PFP subjects demonstrated reduced vasti EMG compared to normal. This was indicative of a quadriceps avoidance pattern which was accomplished through a reduced velocity rather than a kinematic adjustment at the knee. Such a compensation reduced the forces acting on the patella while preserving loading response mechanics.