recent research suggests that rather than VMO weakness it is decreased Glute recruitment that is more of an issue. The Glutes control not only the adduction (weakness giving a Trendelenberg) but more importantly in knee pain, femoral external rotation. If some one has pain on squatting and you teach tham to activate their Glutes you can often alleviate some or all of their pain. Gluteal re-education is a key component of treating PFJ pain. Also check the control of the deep hip rotators, they are the "Wikipedia reference-linkrotator cuff" muscles of the hip and rarely addressed!