Hey Thomas, here are a few Ideas.
1. Do you have full Terminal End Range Knee Extension? Kelly has a great Flossing video about distracting the leg with a band, performing a quad isometric and internally rotating the tibia while the femur is remaining fixed with overpressure to gain further knee extension. Here is a quick way to know if you are laking this. Check in supine and long sitting – If you have a huge gap under your knees when you are in a relaxed position chances are you’re lacking that end range motion. So not only do you want full extension of the knee, but you also want to have about 5-7* active hyper extension – this depends upon your body type. But here is how to check that – In long sitting you should be able to keep your leg on the ground (not performing partial straight leg raise), activate your quad and kick your heel off of the ground a couple of inches. Again its explained very well in the video.
2. Do you have full Hip Extension ROM? If you are limited in Hip extension you can get stuck in a all the time slow death position of slight hip flexion which will also result in knee flexion. A result of this is that it puts a ton more stress on your patella and quad/patellar tendons. Even worse is that it increases the wear down of your cartilage on the back of your patella. So fix it if this is a probelm.
3. Patella not gliding well- mainly up and down but also side to side. If the patella is not tracking up and down correctly it can cause it to do messed up stuff and get angry and be the cause of limited Knee ROM for both flexion and extension.
4. Intra quad/hamstring/Adductor intra muscular movement? – Are your muscles in these regions tacked down on each other or are they gliding smoothly over one another? If they are stiff/stuck/tacked on each othere then they will be painful when you do soft tissue mobs on them. Same goes for the muscles below the knee.
5. With my patients I don’t worry so much about “Isolating the VMO” to help with patellar tracking. Reason why is that all the current PT research shows you get the same benefit of activating the VMO with regular quad strengthening. They are a group and groups work together. So don’t worry so much about Isolating the VMO. Be much more concerned with body mechanics like locking in your ankles through your hips creating troque force. Body mechanics will be huge for you. Strengthen with proper technique and you will me in a much better position.

These are just a few quick thoughts to cover. Really there is a lot more that plays into it, but I hope you find these few tips helpful.

Drew Cook